There was a time about 15 years ago I ran into an old codger who was a retired Air Force engineer. (Shoot, he may have pre-dated the Air Force, but a military engineer anyhow)
He told this story, paraphrased:
"Back in the 50'd or 60's, we were experimenting with a flying disc -- kind of like a plane with no wings. My job was to run the controller, which was a lot like a modern video game joystick. I went up in one with a test pilot. We ran our tests, and when we were done, he asked if I wanted to fly over the nearby town just for fun. We did, and the next day the newspaper said "UFO sighted in town". ..."
I wonder if 99% of UFO sightings are just experimental military craft that never make it to production?