I'm doing engineering in the instance.

Xie Chengzhou thought about the verification method for about two minutes.

The simplest method is to wait for the security guard to walk away, go around his blind spot, and then continue towards the target. This is the method most people would choose—avoiding risk and minimizing contact with the uncertain. But this method has a fundamental problem: it doesn't tell you exactly how the thing senses the threat. What does "10-meter sensing range" mean? You circle around it, and it doesn't react, but you don't know if it's because you went out of the sensing range, because it just happened to turn around, or because it doesn't rely on that sensing mechanism at all.

The information content is zero.

He had encountered a similar situation in Bangladesh: geological survey data led to two completely different interpretations. The construction team's proposed solution was to "incorporate both possibilities for redundancy." Xie Chengzhou reviewed the proposal and said, "No, conduct a supplementary on-site survey first to narrow down the variables before developing a solution." The construction team said that would take three more days. He said that three days would yield a definite number, not two uncertain ones.

This logic applies to abandoned chemical plants as well.

He needs a definite number, not a vague inference that "may be visual, may be auditory, may be something else."

Verification plan: While maintaining absolute silence, actively enter the factory monitor's "visual coverage area"—that is, directly into a position where it can see you—and then observe whether it reacts. If it reacts, the visual perception is valid; if not, the visual perception can be ruled out. This is the most direct test design, with a single condition and a clear conclusion.

Risk: If visual perception is valid, he will enter the perception range, and the experience will fail immediately.

He wrote down the risk in his memo and then assessed it: the factory supervisor was currently in the west corridor, and the route wouldn't turn southeast for about two minutes and thirty seconds. He was in the southeast blind spot, about twenty-five meters away from the supervisor, giving him a good window to conduct a test before the supervisor finished its current section.

The risks are manageable.

He closed the memo and began moving westward.

————————————————————

His method of movement was something he honed in the mines of Africa: he would land on his toes first, absorbing most of the sound, then slowly transfer his weight to his front foot, waiting for confirmation from the ground before taking the next step. The speed was slow, but the sound was extremely quiet; basically, only he could hear it. When he was in Bangladesh, workers mocked his walking style, saying it resembled a crab, but he didn't explain. Later, during a nighttime safety inspection, he was the only one who completed the entire movement on the scaffolding without triggering any vibration alarms.

He moved about five meters west, then stopped, adjusted his position, and found a line of sight that the factory supervisor's current route would directly pass through—from the angle of its route, he should be standing exactly on the extension of the walkway, without any obstruction.

The factory supervisor was still in the western section, with his back to him, walking towards the north side.

He stood still, waiting.

About a minute later, the factory supervisor completed its north turn, changing its route to east and beginning to advance along the north walkway, heading towards his location. It was still twenty meters away, but it was already moving in his direction, and its line of sight—if it had vision—would cover his location in roughly ten to fifteen seconds.

Xie Chengzhou did not move.

He planted his right foot firmly, distributing his weight evenly, and remained still. He slowed his breathing, minimizing the amplitude, and mentally timed it: ten seconds, twelve seconds, fourteen seconds—

The factory supervisor walked past that point where their lines of sight intersected.

It didn't stop. The route remained unchanged, the speed remained unchanged, the two-second rhythm of each step remained unchanged, as if there was nothing there at the spot it passed. Xie Chengzhou waited until it moved out of the line-of-sight intersection and passed the nearest node before noting down his first conclusion:

Visual perception: No.

He retreated to the southeast corner, a blind spot, pulled out his memo, and wrote: "Factory supervisor perception test - Round 1. Test conditions: Distance approximately 12 meters, frontal visual coverage, stationary, no sound. Result: Factory supervisor did not react. Conclusion: Visual perception can be ruled out."

Then he wrote below: "To be verified: Auditory perception. Test plan: Under the premise of controlling the sound, let it perceive the sound signal and observe the reaction."

Then he paused, thought for a moment, and added a line at the end: "Note: The test needs to produce a controllable sound, while ensuring that the personnel can evacuate to a safe distance if the factory supervisor becomes alert."

He looked up and began to plan his next move.

————————————————————

The north staircase is the only feasible route to the second floor. He had already confirmed this in the scan in the first chapter: there are three staircases in the factory. The one in the southeast corner is inside the chemical area, the floor of the one on the south side has collapsed, and only the one on the north side is basically intact, and it happens to be in a blind spot for the factory supervisor.

To get from the southeast corner to the north stairwell, one needs to pass through the central area of ​​the factory building. The central area is an open corridor space with no obstructions. He estimated the route to be about twelve to fifteen meters long. There is a chemically exposed orange-red area in the area he will pass through, which is not on the main route, but he will pass its edge. If he walks along the south wall, he can keep the distance from the chemical area at least two meters.

Two meters should be enough.

He began moving along this route, close to the south wall, controlling his steps and remaining silent, while using his peripheral vision to monitor the factory supervisor's current location—it was in the east corridor, still a circle away from the north stairwell, with plenty of time at the window.

He walked to the middle of the factory and paused for a moment.

The response from the ground was different from what he expected.

He shifted his weight back onto his heel, stepped on it again, and felt the acoustic texture of this floor slab: it was hollow. Not all floor slabs were like this; only this one felt light underfoot, and the sound seemed to spread downwards, clearly different from the floor slabs next to it. It was hollow; there was a cavity beneath the floor slab. He marked the location, walked around it, and continued north.

Then his foot stepped into the edge of the chemical area.

It was just a small step. He glanced down and saw that the sole of his shoe touched the orange-red edge about two centimeters wide—not completely inside, just the edge. He figured that this small area shouldn't have much of an impact; his weight was already shifting forward.

The factory supervisor's pace suddenly changed.

He immediately stopped.

The rhythm of two seconds per step quickened, the interval between each step shortened to about 1.2 seconds, and the speed increased by nearly double. Moreover, the route was off-center—he could not directly see the location of the factory supervisor, but he could feel the direction of the low-frequency vibration from the soles of his feet. The source of the vibration was moving in his direction, and the amplitude was greater than during normal patrols.

He stood still, calculating the distance.

The surveillance system is about twenty meters away from him. If it maintains its current speed, it will reach his location in about twenty-five seconds, at which point he will be within its detection range. If he moves now, the sound of his footsteps will further reveal his location; if he remains still, whether stillness will bring it back to normal depends on the duration of its "alertness".

He chose to remain still.

Ten seconds passed, and the vibration frequency did not decrease; the factory monitoring system was still approaching.

Twenty seconds later, the factory supervisor's footsteps could be heard directly; they were no longer just vibrations, but real, increasingly clear impact sounds, with intervals of about one second.

His throat tightened. Not from fear, but from adrenaline—he'd had the same feeling when he experienced a cave-in warning in an African mine, his body preparing before his brain could even process the information. He labeled this feeling "current distance: danger boundary" and continued calculating.

Xie Chengzhou took a step back, silently moving his feet completely out of the chemical area. Then he pressed himself against the nearest pillar, keeping his back against it, and remained still. He could feel his heartbeat traveling along his back against the pillar, faster than usual. He minimized his breathing, controlling the rise and fall of his chest to an invisible range.

The factory supervisor paused about fifteen meters away.

Then, very slowly, the rhythm began to drop—one second, one and a half seconds, two seconds—returning to the normal patrol rhythm, and the route stabilized, no longer deviating from his direction.

Xie Chengzhou waited until the factory supervisor completed a full patrol cycle and the route returned to its normal trajectory before he moved again, removing his back from the pillar.

He pulled out the memo, his hand steady.

The message reads: "Chemical Area: Stepping on the edge triggers a factory supervisor's alert. Trigger distance: Approximately 20 meters (triggered when the stepping point is approximately 20 meters from the supervisor). Rule 3 specifies a 10-meter detection range; this area is outside that range yet was still detected. Anomaly."

He drew a line under "abnormal" and then started pushing.

At a distance of twenty meters, the sound should normally be outside the range of perception. However, stepping on the chemical area triggered an alarm, indicating that the sound traveled more than twenty meters, meaning its radius exceeded the normal ten-meter limit. The medium in the chemical area differs from ordinary concrete slabs; the dried, crystallized chemical layer may have altered the way sound waves are conducted, causing the sound to travel further.

In his memo, he wrote: "Hypothesis: The chemical zone amplifies the radius of sound propagation, the multiplier to be determined."

He then planned the next verification.

————————————————————

The verification required a control group. Using the same sound intensity and relative distance, but once inside the chemical area and once outside, the plant supervisor's reaction was compared. He found an open area approximately equidistant from the edge of the chemical area, and after the supervisor's route returned to normal, he stepped on it with the same force—slightly more force than when stepping into the chemical area—to ensure a higher sound intensity.

The factory supervisor did not respond.

At the same distance, the sound intensity was greater, but stepping outside the area did not trigger any alert.

In his memo, he upgraded his hypothesis to an inference: "Implicit rule C confirmed (preliminary): Chemical zones amplify the sound propagation radius by approximately 2 (10 meters → 20 meters). Mechanism: Crystallization of dried chemicals alters the acoustic conductivity of the floor slab."

Then he put the previous record about visual perception together with this one, and wrote a summary line above them:

"Factory monitoring mechanism: auditory perception, not visual perception. Chemical areas are acoustic traps, doubling the actual perception radius. Route planning must avoid the entire boundary of chemical areas, not just the center of the stampede."

He closed the memo and took another look at the distribution range of the orange-red area.

When he first entered the realm and saw this area, his initial assessment was "Do not step on it, chemical residue," a safety-related judgment, not a rule-related one. Now he knows it's not just a chemical problem—it's an acoustic trap, and the trap's boundaries are larger than it appears, because the chemicals have diffused along the floor slab gaps. The edge area may only appear to be one or two centimeters to the naked eye, but the acoustic effects have already been activated within those two centimeters.

This trap was designed with great precision.

Xie Chengzhou stood by the south wall, scanned the boundaries of the orange-red area on the ground, retraced its actual outline, and noted it down in his memo. Then he replanned the route from here to the north staircase: instead of walking along the south wall, he would shift three meters to the west, completely avoiding any contact with the edge of the chemical area, even if it meant taking a longer route.

He took a quick note: there were about fourteen minutes until the next route change, and the factory supervisor was currently on the eastern section of the north walkway.

There is enough window of opportunity.

He put the memo back in his waist bag, adjusted his footing, and began to move towards the north stairwell.

This time he took a new route, bypassing all the chemical zones, taking about four more steps and less than thirty seconds longer. When the stairwell came into view, he mentally noted it down: from the southeast corner (a blind spot) to the north stairwell, the feasible route would take about two minutes in total.

He stopped at the top of the stairs, and instead of going up, he put his ear close to the stair wall to listen to the direction of the vibration.

The second floor was quiet, with no signs of activity.

Then he shifted his gaze downwards to the first step of the north staircase—a cantilevered structure with single-sided support. Whether it would make a sound when stepped on depended on the degree of aging of the support joints. He lightly tapped the edge of the first step with his toe to feel the vibration feedback.

The slight outward tilt, about 0.3 to 0.5 degrees, is the result of accumulated displacement due to aging, but the structure is still usable.

In his memo, he noted: "North staircase: usable. The cantilever structure has a slight outward tilt, so the center of gravity is on the inside when going upstairs, reducing lateral forces on the support nodes. Footstep noise risk: to be tested."

Then he closed the notebook and checked the time on his watch.

The plant supervisor had about eleven minutes on the current route, giving him ample time to reach the second floor and complete his inspection before the route changed. He recalculated the safety rating for ascending to the second floor from here, using two now-established rules—the plant supervisor relies on hearing, and the sound level doubles in chemical areas.

The conclusion is: it is feasible, provided that the footsteps are controlled during the process of going upstairs.

He stepped onto the first step, shifted his weight to the inside of the wall, and began to climb the stairs.