We all know that at least some so-called 'UFO' sightings are actually glimpses of military black projects.
Secret, highly experimental aircraft are almost certainly still out there. But fewer than there used to be. Budgets are tight. The military doesn't spend like a drunken sailor anymore. Military-industrial contractors have to fund their own research.
And overseas customers are far more important than they used to be. The US might be keeping the F-22 to itself, but the F-35 can't survive without lots of foreign buyers.
So these days, a new technology is as likely to end up in a promotional video as in the darkened skies above us.
But I bet that won't stop some of these weird and amazing craft from being mistaken for alien spacecraft.
For example, take Lockheed Martin's Various UAV, which is only at the animated marketing video stage (officially at least. I bet there's one or two flying).
In spite of the swept wings, it's a subsonic craft. Those wings have long chords at the root to accommodate vertical lift fans. It's an amazing piece of kit. But alien it ain't.