A seatbelt that's been left outside a car when
the driver closed the door. The metal end flaps and clunks against the side of the car. If enough belt is outside the door, the metal buckle drags along the road
throwing up an occasional spark. More common occurrence in older land yachts with
old school style seatbelts. Driver is typically clueless and doesn't hear it or doesn't figure out what that clunking noise was until he's parked and opens his door.