Cats are famous for their love of boxes. If they can possibly squeeze themselves into a box, they will. Rabbits are the same. Leave out a box, particularly one with a 'roof' on it, and there will be a bunny in that box in no time.
When we're not sure where Trouble is, the first thing we do is check all the boxes. If he's not out and about getting into things or hanging out with the family, that's usually where he is. Rabbits are awesome.