Ok...your goal is to have them come back to a place that is 'untrashed', be it before they left or shortly thereafter.
So...cheat. They come back after they left (avoiding time travel paradoxes) - and their place is untrashed...no explanations given, leaving them to wonder *who* fixed it up...or why. Let them ask the questions, but don't give them answers. Preserves a bit of mystery for the next book.