>>19630
This is the door for "store" stage. Developers put inside some parts of the game like two monsters model, a train, a bed, etc...
You can still play the wave of monsters opening a common door.
I don't know how to open it but I guess you can do that after collect all the maps with spots for poo/masturbation (5 pieces on 2nd whiteboard inside tutorial room). I unlocked it without kill many monsters, without collect other porn pages and without drink much water. Only completing maps spots and after ate 50 pieces of chips/apples.