In the Shrieking Shack when Lupin and Black Zap Scabbers to make turn back into Peter Petigrew, he gets wedged into the hole fully clothed. Later on by the Whomping Willow when he turns back into a rat all his clothes fall off.

Where does he keep these clothes whilst he is a rat? I realise that they wouldn't show nudity in this film but it annoys me every time because when he initially turned himself into a rat all of his clothes would have fallen off then as well.

Maybe the book explains it?