The UFOs could be autonomous, or they could originate from within our solar system.
As for within our solar system, I'm fairly certain this isn't true. There would be a crazy amount of effort to hide all that. Like you're hiding an entire civilization from x-rays, visible light, infrared, and radio. A civilization that would be constantly monitored (we pay a lot of attention to local objects). Possible, but these aliens clearly spent a lot of energy and resources to undertake this but have also hidden that resource collection (there's a feedback problem here). Combined with that we aren't sure for mechanisms for life. I'm not going to hold my breath.
Also, I don't think we can rule out aliens within our solar system. An alien civilization beneath the ocean of some moon?
To... what? Study a civilization and do nothing with the data?
> Also, I don't think we can rule out aliens within our solar system. An alien civilization beneath the ocean of some moon?
Yeah, we pretty much can. Odds are extremely low. Now, life might exist, but not advanced space fairing life. Those civilizations produce a lot of noise and make drastic changes to their planets (/moons).
Arguments like this neglect the relative inscrutability of even human motivations, with which we ought to be intimately familiar, yet can nevertheless perplex us.
Humans undertake spectacular projects as jokes, religious activities, and as art. There's really no speculating about equivalent, befuddling alien motivations, and certainly nothing to rule out autonomous devices that travel unfathomable distances, and then play hide and seek when approached directly.