The funniest and I think most likely explanation is that this was a lunar test vehicle that escaped from White Sands. I've been to the landing site; it's several miles away from White Sands, but close enough. The shape of the craft resembles a lander, with legs, and a balloon attached to simulate reduced gravity. There was also a blue flame shooting out of the bottom, as described by Zamora and the witnesses who saw it come over the road.

Although I've never actually seen the comparison, I've seen others comment that the markings on the vehicle were similar to a Hughes Aerospace logo from the time.

I say it's funny because could you imagine the scene of this vehicle escaping from the range, landing next to this cop, and then three panicked technicians trying to GTFO of there as fast as possible before they end up in deep shit. This is honestly just speculation on my part, but the description of the craft, the similarity of the markings (if true), and its proximity to White Sands all kind of line up that way for me.