Oh the stealing part, totally- Though he did seem genuinely confused when the whole "People's stuff is going missing" started (He wasn't even inside the castle when it started). Also, it's happened before a long time ago according to one of the maids, hence the ghost blame-age. Could very well be him and the author is purposely making it look impossible. Though he is currently using that maid for information-
I'm sure the "intruder" and the person who stole the items aren't the same person/thing. But if they are I would want it to be a ferret. lol

I know it's probably a cat or a dog, but I really want it to be a ferret. Since ferrets are known to steal things... It's more likely to be a dog or cat, or some kind of bird that's collecting things for it's nest...