How are they verifiable? Whose going to step up and say "Oh yes we have a top secret military craft we were testing that we were hoping to keep secret from you guys"? As for the plane off course, I would think that as a pilot, if you see something you think shouldn't be there the first step would be to rule out other planes and so verifying it as a plane not where it is supposed to be is what starts to make it a UFO, what seals the deal is if you can't make radio contact. U = unidentified, not unkown although it might be unknown as well as unidentified. If the pilot seeing them was sure it was a secret military craft or a plane off course he wouldn't report it as a ufo. you seem to be operating under the assumption it gets seen by the pilot so clearly that he can analyze it in great detail.
Now this I agree with and I hope you realize I'm just being fair, you seemd to be jumping too quick for the banana technology option
if you want to keep an airplane secret, first, you are not gonna fly it above populated areas, much less on airplane routes, which are easily avoided, as well as any other place where there are other planes, since you have radar.
lets take my sighting as an example, this was in the Venezuelan Andes, in a city, which is in-between a bunch of mountains, and around the city all you have is mountains and more mountains, mostly national parks and almost uninhabited.
so if what I saw was a secret military test plane, why fly it directly over a densely populated area? why not just fly it over the national park mountains? save the chance of anyone spotting it?
why would you fly a highly advanced technological plane close to a regular run of the mill plane when you have sophisticated radars designed to detect other airplanes if you want to keep your secret military plane secret?
makes no sense.
also, a pilot when seeing some aircraft where it should not be, does his best to identify it, since there are rules and regulations in the aerial space, and if anyone is infringing those, they are subject to fines and other sorts of punishable offenses; since doing such endangers other planes who are indeed following all regulations.
so if we really understand this, saying any ufo should be secret military or merely a pilot's way to file away a plane that should not have been where it was, such justifications also fall way short.
so we're either left with extraterrestrials or banana-tree tech, since we can easily identify what these ufos were not.
peace