stealing

On general principle, cats have no respect for you or your things and they'll knock your full mug of coffee off the counter just to see what will happen. It makes sense that there'd be a lot of evidence of cats and their crimes. Hey, they want food even when they have it. This point is driven home by the fact that cats probably talk like disparaged Victorian children.

The only good thing about having a roommate that steals your stuff is that if you tamper with your own stuff, they're in no position to complain about your stuff that they deny taking. This is a pretty common tactic. A similar and more explosive thing happened when this food thief's roommate switched to regular milk.