Possessed Mannequins
This theory is a little out there, but hear me out. Everyone remembers the infamous mannequins from Hello Neighbor Alphas 1 through 3 and how they seemed a little... strange to say the least. What with the school room students, the buried teacher, the mannequin the Neighbor tries to capture in the picture listed, the small school area in alpha 3, and the knocking in the blue key room. Here's what I'm presenting: In the early alpha days it was confirmed that the Neighbor was basically the devil trying to keep the player from ascending and lock them in hell. What if the mannequins are the Neighbor's past victims? Every instance where the mannequins seem alive: its because they ARE. I think the biggest piece of evidence for this would be the fact that the teacher mannequin is buried in the grave: the same way the Neighbor gets rid of both the Prototype victim AND the Alpha 1 victim, the SAME build we find the buried mannequin. Maybe not the best theory out there, but just something I thought of.