My father-in-law did his national service as a radar operator at RAF Buchan in the north of Scotland in the 1950s. He told me once that one evening he saw a plot on his screen that was moving far faster than any of the jet fighters were capable of at the time. He immediately reported this to his supervising officer who told him to ignore it. This type of incident was infrequent but far from unknown. He wasn’t a fanciful man, a typical product of his time who had a long career as a draughtsman. He was also skeptical that it a was a UFO, his view was that someone (the Russians, the Americans or us) had technology at that time considerably in excess of what was then publicly known.
This probably offers the most convincing explanation. Seeing things we don’t readily understand does not prove the existence of UFOs.