No there is no mistake when they re-appeared they were still in their night clothes or in Monica's case just bra top and pants.
They were in there all right and this has happened on more that one occasion so it isn't a one off.
What were they doing in there you ask, well your guess is as good as mine but they certainly wern't humping loads of washing around.
You believe what you want to Mee4you but i can only relay what i see will leave any other thoughts up to you and others.