i don't know. i really can't think of anything that sounds remotely plausible. it drives me crazy. the obvious explanation seems like secret military aircraft but it seems unlikely for a bunch of reasons. they usually want to brag about technological advancements, they show off every minor improvement on regular aircraft. it would be really weird for them to fly them in a place where they'd attract this much attention. and that's ignoring how long ago this was. i never think about ufos in the context of aliens, i think it's extremely unlikely that they've been here, but it somehow seems like the most likely explanation.