DRIVER STARTED CAR TO COOL IT OFF BEFORE RUNNING ERRANDS. IT WAS PARKED ON THE FLAT PORTION OF THE DRIVEWAY. IT WAS IN PARK & I DID NOT TOUCH THE GEAR SHIFTER. I LEFT THE DOOR OPEN TO ASSIST IN COOLING THE INTERIOR OFF MORE QUICKLY AND WENT TO RETRIEVE AN ITEM NEEDED FOR THE ERRANDS. WHEN I HEARD A STRANG NOICE I TURNED AROUND TO FIND MY CAR MISSING. IT HAD REVERSED ITSELF 180+ DOWN THE DRIVEWAY, ACROSS OUR LAWN & ONLY STOPPED WHEN IT HIT A 24'+ TREE. WHEN I REACHED THE VEHICLE THE GEAR SHIFTER WAS STILL IN PARK. INLINE TOWING HAD TO WINCH IT OUT, AND NOTED THAT EVEN THOUGH THE HAD SHIFTED IT TO NEUTRAL THE BACK UP LIGHTS WERE ENGAGED. IT HAS BEEN TOWED TO THE AUTO BODY SHOP FOR REPAIRS. INSURANCE HAS BEEN NOTIFIED.