WE PARKED AND EXITED THE VEHICLE (TURNING IT OFF) ON A SLIGHT GRADE AS WE NORMALLY DO IN THE FRONT OF OUR HOUSE ON A PUBLIC STREET. WE AWOKE THE NEXT MORNING TO DISCOVER THAT THE VEHICLE HAD ADVANCED OR SLID FORWARD APPROXIMATELY 15' COMING TO REST AGAINST A TELEPHONE POLE ADJACENT TO OUR CORNER PROPERTY. IF THE TELEPHONE POLE WAS NOT IN THE PATH OF THE VEHICLE IT WOULD HAVE CROSSED A SIDE STREET AND COME TO REST IN OUR NEIGHBORS FRONT YARD. UPON VIEWING THE VEHICLE AFTER THE DRIFT FORWARD, BEFORE ENTERING THE VEHICLE WE COULD SEE THAT THE VEHICLE WAS CLEARLY IN PARK AND TURNED OFF. THE VEHICLE HAS APPROXIMATELY 40,000 MILES ON IT AND HAS HAD (1) PREVIOUS SERVICE BY A DEALER FOR TRANSMISSION SLIPPAGE FROM 1ST TO 2ND GEAR.