It happened after 1:00 p.m. Wednesday at the Wawa in the 2700 block of Pulaski Highway in Newark.
Police say rising temperatures inside the closed truck appear to have caused the air horn to explode.
Earlier reports said the explosion sound heard in the vehicle may have come from live ammunition left on the dashboard of the vehicle going off.
Police now say that although shotgun ammunition was found in the cab, it was an air horn, not the ammunition, which exploded.
The owner of the beige GMC Sierra pickup truck has been contacted and is enroute to the parking lot to retrieve his vehicle.
Police tell Action News the man is not being cited for any offense.
