Students of Hogwarts had perennially hidden from passing prefects in a small, hidden hallway, only visible when your hand touched a certain stone. Prefects, remembering well their own miscreant days (or more likely, those of their friends), seemed to forget the secret.

No one knows who first found this secret: Perhaps it was a witch, marking the height difference between two friends.
Perhaps it was a sleepy first year, resting his head against the wall.
Perhaps a teacher, a house-elf, a visiting parent.

Perhaps the stone had simply offered itself to someone in need.
One never knew, with the castle.