I'm starting to believe that the library might be haunted. Books falling off shelves, strange foot-prints in the dust in the restricted section where the house-elves won't clean.
Of course, it could as easily be students sneaking into places they're not allowed but I'd like it if it was a ghost.
A past librarian hopefully. It seems rather tragic that a student would choose to haunt the library and not, say, the Quidditch pitch or the boys showers in Quillory which I understand from some scribbling in the margins of a second edition of Defensive Magical Theory is the place to sneak a look.