"Rules!"
That was right, the rules!
Take the Teaching Building Rules, for example: [No one other than students, teachers, and patrolling security guards will appear inside the Teaching Building. If you encounter anyone who does not fall into these categories, please return to your classroom immediately. Ignore everything along the way, even if a teacher calls you.]
[...Evening Self-Study begins after 18:00, but this school does not require students to participate unless the current time is past 21:00... During this period, if you see anything abnormal outside the window or someone calls you from outside the door, please ignore it.]
"Anything abnormal outside the window or someone calls you from outside the door," and after 21:00, too-didn't that mean the layers had begun to destabilize?
Throw in a little abnormal weather, and wouldn't it be just like what had happened earlier?
So that was the underlying logic behind this section of the rules!
Following that reasoning, a call coming from behind might not necessarily come from a Night-Walking Species following close behind. It could also come from a mystical creature in another layer amid abnormal weather!
Besides, much like the Cafeteria's layout, once the classroom's bulletproof glass fogged up, it vaguely acquired a "door attribute." To some extent, the drifting, overlapping layers of gray fog also seemed to possess a "door attribute"...
Perhaps the door attribute was some kind of rule that separated layers. The Architects who originally built the Campus might have constructed physical doorways at locations with stable door attributes, then used people's cognition anchors to anchor them from both sides, turning them into stable clusters of buildings.
At certain moments, perhaps a "door" sustained by human hearts was the true door that prevented mystical creatures from invading the realm.
With that thought, Lin Yi lingered no longer