i'm with sprinkles on this one... the majority of these sightings are floating question marks.
that's being said, i do think (well, not a thought of my own, rather i find the sources saying such more reliable), that a lot of historic sightings where current technology in development stages, both success stories like the modern drones & stealth bombers, & massive failures that didn't get passed trial. and more of the later, because there's a good chance that if you see it as a dazzling light, then the point of leaking fuel on the slow burner or melting metal parts has already being reached.
i also think one reason a lot of this crap is unidentified is because its not a finished product, rather, its the basic stuff in experimental stages, the first time a technology is taken out of the lab will rarely look like an actual airplane, and is more likely to be a cheap ass prototype, whether its a new engine (that someone caught burning sideways and swaying like an alien spacecraft), a reusable satellite deploying rocket didn't quite make the way back, an experiment with light materials that turned out to not be capable of taking the pressure and air friction of a sonic boom and exploded to dozens of slowly falling heated debris, an experiment with magnetic solar sail mounted on a satellite that ended up coming not so gracefully back to earth, etc...
in all such cases, both commercial companies and military organizations have largely enjoyed people calling these technologies alien sighting, because if its a successful technology then you don't want the competition to get the idea of what your working on, and if it's a big ass failure you don't want people to realize you've invested 100s of millions of dollars into something that blew in their faces...
this doesn't explain everything, but the reality is that alien conspiracy theorists are largely harmless, and there's no cover up like the cover up of one's own ass.