If some of the larger UFO cases are believed, those would be soundless drones that travel 1000s of mph. Also, to play devil's advocate, what informs you that being unseen is a goal? I don't have an opinion on what these things are, have never seen one myself, but some events have hundreds of witnesses and I don't think drones or debris captures what they describe.

The only way I could figure would be aliens would have no trouble remaining hidden if they wanted to and the sightings are used to condition people to not be as freaked out as if they just showed up one day.

Additionally, most people don't know that weather services around the world release nearly a thousand balloons at once, twice a day, every day. That's a lot of potential airborne trash right there.

[removed]