How the F could this have happened?

I drove my sisters car to school and back. I always park it by the dumpster on one side of our apartment. I went to drive it to school today and it was on the other side of the building. It was unlocked (which I am sure I locked it before) and all the stuff wa messed up. My sisters ipod was gone and everything was thrown all about. That fact aside how could someone have driven her car. It wasn't me my sister or my dad and we are the only ones that live here. We all have our keys and no wires were out of place so how could someone have driven my sisters car without a key?