Professor Snape: A remarkable feat, don't you think? To enter the castle on ones own, completely undetected?
Dumbledore: Quite.
Professor Snape: Any theories on how he might have managed it?
Dumbledore: Many. Each as unlikely as the next.
Professor Snape: You may recall, prior to the start of term, I expressed concern about the appointment of Professor...
Dumbledore: Not a single person inside this castle would help Sirius Black to enter it. No, I'm quite convinced the castle is safe, and I'm more than willing to send the students back to their houses.
Professor Snape: What about Potter? Should he be warned?
Dumbledore: Perhaps, but for now let him sleep.