Johnston Fire was dispatched to a reported garbage truck on fire in a business parking lot. E3970 and C3900 arrived to find nothing showing, with what seemed to look like the fire had been extinguished. Crew of E3970 was directed by command to do some overhaul to verify the fire had been extinguished. Boxes falling out of a large garbage container and landing on the exhaust pipes caused the fire. Overhaul was complete and area was cleaned up.