Thats why you dont invite people you dont know/trust!
Sucks for you then.
I had my own apartment since I was 17. Once, I had a party and some of my friends brought people I didn't know. Someone went in my bedroom drawer and stole a bunch of cash.
Isn't that what happens at every party? Though not always valuable something will be missing :/
Did you write down its ip address so it could be tracked down if stolen. And rule one of parties involving young teens: Lock them out of the rooms, unless you can trust them. Also, do you have a list of all who attended the party. If so, call the police so they can get a search warrant for each if possible.