Casper at it again...

Ok so my resident ghost, Casper, made his biggest move so far. My roomate, Justin, asked if I had gone in his car to get something because when he went outside his windows and sunroof were open. When I replied no, we called our third roomate in and he said he didn't do it either. This set us to pondering how it could have happened and the only thing we could come up with was either Casper had done it, or someone had come into our locked house, grabbed my roomates keys, opened his sunroof and windows, and brought his keys back inside, all without taking anything out of our house or his car (his iPod was sitting in the front seat). We know it wasn't Justin because he hadn't driven anywhere that day (it was around 11am) and the windows weren't down overnight because it stormed that night and the inside of the car was bone dry. Anyone have any other ideas on how this couldve happened?