People are glitching themselfs inside the walls, inside the "unused" rooms in the second floor.
Thus giving them full sight in all rooms and outside. They wait here for their pray to come. Even though they cannot shoot through the invisible walls, they glitch themselfs out in a second, kill, then glitch themselfs in.
I believe if you are standing in the corner by one of the beds and log out, and in again, you login inside the wall