So apparently at some point yesterday Tim nicked Alex (room 1)'s bed mattress... and put it someplace inaccessible (something about needing a chair to get the mattress back, but the chair's locked in the bathroom...?). As revenge, Alex snuck up to the room last night and nicked Tim's mattress, brought it to his room and went to sleep on it.
End result?
Tim slept on the floor last night, and probably will do so for the forseeable future.
Lesson learned:
If you're nicking bed mattresses, it's probably a better idea to be the second one.