[Hellmouth]

I don't know, just speculating that it might be something anomalous/unknown. Remember the hubbub about the sprites?
Perhaps it's a low atmosphere version of a sprite. It might be a missile - but don't missiles either go boom or crash to the ground and leave debris? So, that would seem to rule out the missile hypothesis.

[Gunrunner2]

My Post (#59) has the best explanation.

The radar return could have been a temperature inversion or even a bunch of bats. I'M NOT KIDDING. Our radars can pick up that sort of thing.

Bats, birds, whatever. . .we have a radar mode we can select that will screen out targets below certain speeds.

I used to lock up cars on the highway, just for fun. Of course, NEVER when carrying live ordanance.

[maquiladora]

Yeah, missile debris would have been found.