Sightings of UAPs (Unidentified Aerial Phenomena) by commercial and military pilots have been reported since WWII and have been routine for the last several years.
There are several possibilities. Secret technology by an advanced Earthly nation is one. This is very unlikely. No human security program could be successful for over seventy years. Too, human governments would long ago have bragged about such advances. And they certainly would not be spending many billions today on less advanced propulsion.
They are craft piloted by extraterrestrials. If so, why aren’t they routinely sighted at lower altitudes or from the ground? If their purposes are benign, why haven’t they long since communicated? Seventy years of spying on radio, TV, and the internet should have given advanced aliens the ability to talk to us.
Or if their purpose is malign, why haven’t they attacked us in some way? Perhaps they have and they’ve just been subtle enough not to be detected. Maybe they have been stoking hatred of all kinds. Disrupting our economies. Sowing epidemics and pandemics. That’s possible but we humans seem well able to do those actions ourselves.
A third possibility is that the craft are unpiloted drones of some kind. That would account for reported accelerations and course changes that could kill humans. That would explain why the sightings are mostly at high altitudes. Craft fly better in thin air. That would explain why sightings are of such poor quality. Drones can be quite small.
A fourth possibility is that the UAPs are natural phenomena. Maybe there are electronic life forms in the upper atmosphere, beings made of plasma. Possibly curious about the cold metal objects invading their natural habitat. But also shy about approaching those objects too closely, sending them fleeing back to the realm of the Northern lights, their natural home.