We weren't exactly chummy at the time. I don't think she'd risk the children, though. Not when she knows Snape's out there.

Sometimes I think the old place just doesn't want to be airtight. It knows that sometimes one might need a secret way in or out, and it likes to oblige. Although I never thought it would prove obliging to him, of all people.

Do we know for certain that he hand-delivered this note? Perhaps he imperiused one of the children to take it onto school grounds for him?