Bug: See an item you like just on the other side of the security window? Door's locked? Doesn't matter, it's free real-estate!
You can also drop items through the security window. Don't do that.
Unrelated: Huzzah, I completed my genocide run! Every classmate liquidated, book acquired, exit reached! I got unusually lucky with the keys, so I didn't even need to open several rooms - like the security room, which I checked via the window shortly before the end.