Many times, the Fat Lady — the guardian of the Gryffindor Common Room — sees Harry and his friends going back to the Common Room after a late-night wandering, and she sees that they have been out when they are not supposed to.
So, why does the Fat Lady never tell a passing professor that students had been out of bed at night? She knows it's wrong, as she "tells off" Molly Weasley for being out at four in the morning. So why doesn't she tell professors about these out-of-bed students?
Note: I'm not a hundred percent sure about the Slytherin Common Room entrance and if it is able to communicate, but we don't know about Hufflepuff's entrance to their Common Room and Ravenclaw's knocker can only tell riddles (as strongly implied in DH, when Amycus Carrow tries to break in the Ravenclaw Common Room without answering the riddle correctly), so I've only included the Fat Lady in my question.