Tyler Cowen, always a good source for new weird things to think about, recently wrote a piece claiming most people underestimate the chance of present-day alien contact. He lumps panspermia into this bucket, so perhaps I should be disappointed he's only shooting for a 1/1,000 estimate in his readers. For the sake of this piece, I'm ignoring panspermia – it's far too reasonable to be an "and also" for UFO speculation.
The weird part is, he seems to identify enough serious issues to make the point seem weak. For instance, he admits there's no model of alien goals or behavior that explain UFO sightings. Aliens would be either hidden or more obvious, almost no matter what their goals are. We imagine aliens having different goals. Inhuman goals, even. But they have some goals, and if no goals in the span of human imagination (or insanity, as is unfortunately sometimes the case with alien discussions) would lead to this behavior, it's unlikely the behavior is happening. How could almost totally pointlessness be driving the resources necessary to cross star systems? Those costs tend to sharpen the mind on purposeful behavior.
Of course, there are modern day, well funded (and, not to be rude, but: known to exist) groups that could be producing this behavior. Foreign governments that wish to test American readiness and stealth detection. Different branches of the American military wishing to maintain operational security of mission. And private actors testing weird things that might not even be aircraft. Could even be similar to whatever causes Steve (a mysterious, seasonal aurora effect that emerged recently caused by somewhat inexplicable high-speed, high-altitude plasma), and that's probably some strange accident, or a side-effect of weapons tests done on the edge of space.
At the end of the day, you should probably rule out aliens (at least below the 1/1000 chance threshold Cowen supports) without even considering how, for instance, we haven't seen evidence of alien life. We haven't, but you're almost using too much evidence to reach the conclusion. Being right at the edge of detectability is practically a signed confession saying "I am a modern era human who can mostly but not completely avoid detection". Being detectable yet not targeted by the Air Force is essentially the job of the current Air Force, and we'd expect that. Aliens who could travel star systems would be like a Predator drone targeting ancient Romans – so unseen that they'd only be known for their destruction. The Romans would call them gods.
But these UFOs? They seem more like annoyances.