An explosion before 2 a.m. Wednesday alerted residents in the 1900 block of Sparrow Hawk Street to a fire in a neighbor’s white van parked at curbside in the neighborhood on the southeast quadrant of the 120 Bypass and Airport Way interchange.One witness said he believed the explosion was a pipe bomb. The witness said he was outside minutes earlier having a smoke when he noticed a white pickup truck hanging around in the area near the van. He recalled that shortly after going back into his home he heard a loud boom.