Something for another day, indeed. Not that John's sure he would tell somebody. He doubts a lot of people know where they are and use them, so they might as well get to be there. Hogwarts is full of those things, anyway; secret passages and whatnot. Plenty of teachers probably know at least some of them.

They probably don't do much harm.

He rolls his eyes, just slightly. "Don't worry, I will be," he says, doesn't mention that Sherlock's probably far less careful than he is but it's definitely there in his tone, and then he grabs the handle and pulls the door open.