Hence, these particular "top secret" aircraft exhibit atypical characteristics that are so out of this world that they are being construed as ET spacecraft?
Most UFO sightings don't feature "out of this world" behavior. Usually it's just a weird-shaped object in the sky, or a series of lights that look irregular. If there's something actually acting strangely, it's probably not an aircraft. I recall seeing a UFO show the other night that had one case about a city-wide sighting that turned out to be some kind of balloon drop or some such thing. I honestly don't remember the details, but the footage looked pretty impressive. It was dark, the lights were on different levels; it really gave the impression of a single, large object very slowly descending to the ground.
Oh. So now we're talking about regular flying aircraft doing regular flying?
When weren't we? That constitutes most of the UFO sightings I've ever heard of. But even if we're not, without a frame of reference, there's no way to say with any certainty that the motion they think they're seeing is the actual movement of the object. Something moving very slowly away from you can appear to be stationary, for example. If there's no reference, there's no way to tell. And you'll notice, any time we get a sighting of a "craft" exhibiting odd impossible behavior, it's at night, when the best you can make out are the lights. That's no coincidence.