Good point, unless Rose also had invisibility powers. In most of the cases I'd say we don't know for sure how long she had been there before she ran into Donna but the one where she tells Donna to use her raffle ticket (which could almost be a whole point on it's own probably) she didn't say that and then stand around twiddling her thumbs for half an hour waiting to to zapped back did she? No she came in, left a message, and got out.