What you need to do is talk to her about it. It would be totally uncool to dump her as a friend if she didn't do it. Besides, if you keep quiet, you're always going to be stewing inside. Tell her you've noticed a few things missing (be specific about the items) and ask her if she borrowed them. If she says yes, tell her to ask first, 'cause you can't trust her if she just takes your stuff. Make sure she returns everything. If she denies it, pay close attention to her when she's in your room. If it happens again, tell her you know she's stealing and that she needs to find a new friend.