2005 Chrysler Sebring Fan continues to run

This morning when I went to start the car, the battery was dead. I thought that this was very strange because the lights weren't left on nor was the door open. I got my jumper cables and used my roommates car to jump start the car and get the battery going. As a charge transferred from his car to mine we heard a fan turning but the keys weren't in the ignition. I thought this was very strange seeing as the car wasn't on. I drove to the store and back and the fan wasn't on when I went into the store or when I got home. I went outside about an hour later to pull something from my car and the fan was on again. Could this be caused by a short or something like that? I can't seem to figure out what would cause a fan to be on without the car itself being on.