Awww, that sucks Kee and like Erin says- I hope makeup karma comes and bites her in the ass if she did take it!
Since I'm a mark rep I host mark parties and I am ALWAYS making sure all my stuff is still around, even if it is the demo products. Usually it's always friends and family, but you still never know and I don't want to accusse anyone of stealing, so I just have to keep an eye on it all. Some of my friends do have *sticky* fingers or will *borrow* stuff without even asking, ughh I hate that!