I think she's the kind of person who will try to get away with something for as long as she can. Certainly she knows its wrong to take your food, but figures that if you're not saying anything, then why not take your food instead of hers?

Watch out for shampoo, hair gel, perfume, etc levels. My one roomate in college liked to use my stuff instead of hers simply because she could get away with it.

You've had the conversation. Let it go for now and watch things.