I like the classic "1000 cups of water." Just wait for him to come back, then unscrew his door handle and place the four screws at the bottom of selected cups. Then, place as many plastic cups of water as you can afford, filled up to the top all around the flat (where you don't need to go obviously), in his room, on his floor, on his desk and outside his door. Then leave him a note saying that the screws to his door handle are at the bottom of one of the cups outside his door and he has to drink it all the cups to find the screws so he can get back into his room.
Wouldn't he just look into the cups of water, see which ones the screws are in, then just either drink or empty those 4? Or am I missing something?
