While he waits for someone - whoever - to enter the dorm, he casts a series of quick charms on his trunk and side table, where he keeps his own clothes and personal items. He should have done this sooner, but these will tell him if people go snooping. Now that whatever this is with Crabbe is bubbling to the surface, he needs to be careful. He doesn't have anything in here to hide but he expects someone to pry through his things nonetheless.