Luke has known for a while that there is someone in the school watching over him and performing silent spells on him so he doesn’t miss the bottom stair and land on his face or lean too far over a banister only to fall and break his neck. It took him longer than it probably should to work it out, and, actually, Luke doesn’t actually realise himself until Ashton points out bluntly that you can only do a trip-fall-sprawl without actually touching the floor a few times before it’s obvious someone has their wand pointed at you, and it’s not just Peeves being abnormally friendly.