>(Also, this is the US military messing with itself. Someone in some skunk works hanger in Nevada has some secret weapon and they want to test it out on a real navy.)

This would seem to be a pretty likely explanation (near-infinitely more likely than ETs, at least). It'd also perfectly explain David Fravor's story of spotting strange vehicles during a Navy training exercise (e.g. how the craft seemed to have advance knowledge of a Navy meeting spot, and the reaction from his superiors).

He thinks it couldn't be US military, but if it's some covert/intelligence thing, doesn't seem too implausible that they may try to do a realistic, unannounced red team exercise during a routine training exercise. That wouldn't explain his claims of it performing gravity-defying maneuvers, but there's no evidence of that besides his and some of his colleagues' testimonies, though.