As a pilot, I get the chance to see a lot of unusual things "up there". When you fly in the mid 40,000's you are also free of most light pollution, temperature variations and atmospheric contaminants.
I have seen ball lightning, thunderstorm sprites, satellites, the ISS, meteor showers and what was either a gamma ray burst or one supremely timed meteorite (did not change direction at all, nose on while flying 550 mph.)
A UFO? Let's just leave it in my not-yet category.
Jim