Hubby & I were just discussing the same thing.
Apparently, she wanted to be w/that one boy and after the classmates & chaperones turned in for the night, she slipped out and met the boy and his mates.
Her passport, handbag, id, et al were in her room and her bags were packed to go home the next day. (Shades of Chandra Levy and her disappearance.)
I didn't realize she'd packed up and was ready to meet the group to head home in the morning. Sounds like she planned to party all night.
Leaving a passport in your hotel room is an extremely poor thing to do while traveling internationally. The only time you should be separated from your passport is when you are staying at a hotel where they take it for safekeeping at the desk when you check in. Preferably they should be carried in a zippered pouch of the sort that is worn under your clothing.