Hmm... Is there any one who comes and goes often that could have a motive?

Ichirin, Murasa and Kyouko aren't really suspects.

If this were Kogasa's doing, it would be quite the surprise, but she doesn't have the brains for it.

Koishi could've taken it subconsciously...

What do you mean "taken" it...

I won't get my hopes up, but what is it?