VEHICLE WAS PLACED IN PARK, SHUT OFF, AND EXITED. VEHICLE WAS PARKED ON A HILL. SEVERAL MINUTES LATER VEHICLE STARTED ROLLING DOWN A HILL; NARROWLY MISSING A YOUNG GIRL. VEHICLE CAME TO REST ON ITS OWN WITHOUT SUSTAINING DAMAGE. VEHICLE SHOULD NOT HAVE BEEN ABLE TO MOVE WHILE IN PARK AND SHUT OFF.