This is the most horrible thing I can imagine! What kind of horrible person would be so spiteful??? If you can, take lots of pictures of it, as clear as possible and absolutely inform your host. I don't know if during the party your bag was stored in a common area with coats, etc...and if so, I'd ask other guests if they noticed anyone paying particular attention to your things.
I would do everything possible to discover who did it and expose them, because that is simply unacceptable, unforgivable behavior!