I realize that it may very well have been a guest. I did have my kids send an email to all in attendance, inquiring if someone may have "borrowed" it to get home and forgotten to return it (as another way of saying no questions asked), if they saw anything, etc. While I don't think it was someone in their inner circle, I know these kids well, it easily could have been a friend of a friend that was legitimately there.
Even though this wasn't MY bike (although it is a $ loss for us) I think I am so upset that it happened inside my own home.