Among the many fraudulent stories about crashed ships in the possession of the military are some stories that seem real. The local populace makes numerous observations, and there are tell-tale signs that remain for the curious explorer to see long afterwards. Well, in these situations, something is going on, it's just that it's not of an alien origin! As a result of their early agreement with aliens in the Service-to-Self, the military got possession of several ships. This is certainly no secret, as Bob Lazar confirmed the rumors. The military has never figured out how to operate the ships they were given, and many deaths have resulted from their attempts. Unlike human vehicles, which have a propulsion system and manual controls, alien ships house a strange device that cannot be examined and mental controls.
Eventually enough control was attained that the ambitious generals would order a field run with this ship or that. The idea was to do more with the ship than simply lift and wobble about. They wanted rapid travel across the skies. They wanted to impress the heck out of anyone they were interested in intimidating. At least they could do that, they reasoned. So the human pilots, selected for their confidence and demonstrated skill, would aim a little higher, a little farther, and WHAM! The crash remains, which might be found anywhere, would be quickly collected by special military teams scrambled with the greatest haste. The most likely excuse would be given to the media and local authorities, and since the crash site is always thoroughly cleansed, countering this story is difficult.