In preparation for the personal flight, the pilot started the airplane to warm the engine. He then set the airplane’s parking brake and exited the airplane to load baggage while the engine was operating at idle. He asked the passenger, who was not a pilot, to board the airplane while the baggage was being loaded and to pull the mixture control if anything should happen. The pilot then went into the fixed base operator to get his debit card and to use the bathroom. Within two minutes, he heard the engine sound increase, and the airplane taxi into an unoccupied vehicle that was parked about 75 yards away. The airplane sustained substantial damage to the outboard section of right wing. The pilot reported there were no mechanical malfunctions or failures with the airplane that would have precluded normal operation. The pilot stated that the accident could have been prevented by never placing an unattended non-pilot passenger in parked operating airplane and would be scheduling remedial pilot training with a flight instructor.