My theory is that these are not aliens but rather time travelers from our distant future. Kind of a 12 Monkeys idea, coming back to the points in time we really almost screwed the pooch. But that said, they don’t seem to be too much concerned with the Time Travel Paradox if they let us see them. Perhaps the proof will be sightings clustered around specific locations. Any sightings over Wuhan?
That's what this allegedly famed astronaut suggested as well; decades ago I saw Hynek give a talk -- he toured behind the Close Encounters movie -- in which he suggested "metaterrestrial" as one of the possibilities. By that term he meant, Earth dwellers from the future coming back to have a look at us. Of course, that doesn't solve the distance problem, because in even 1000 years the Solar System will have moved a long way, and the galaxy will have moved off in some direction. Our Solar System moves about 500K mph, for reference. So, it isn't much of an explanation, introducing as it does a new problem while failing to solve the old one.
Time travelers returning to a date centuries before them to pick up heirloom DNA to treat mutations rife in their period.
Pretty cool SciFy concept, no?