I'm very much in the 'moving other peoples' things is worse' category. It is a petty action that could very easily have resulted in a much greater confrontation, and I would be rather miffed if someone were to mess around with my belongings in my absence whilst I popped to the restroom or to get a coffee. To say you didn't know where he was going seems rather disingenuous - he was obviously not leaving the Lounge, or he would have packed up his belongings.

I'm not saying that the person in question was not selfish and acting in an unbecoming manner, but the way the OP dealt with it is inappropriate.