The truth is out there. The Japan Airlines 747 UFO encounter near Anchorage is pretty interesting, November 1986.
Knew a guy that did radar duty back in the day. I believe he was somewhere in Maine.
He related experiences where they observed high altitude “fastwalkers” that would make incredible high speed maneuvers and leave the airspace as soon as anybody was scrambled to take a look. Sometimes they’d return as soon as the plane was back on the ground.
He was a believer. Use as many grains of salt as you’d like.