> Autonomous devices wouldn't need to send signals back home. They might be fully autonomous.
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).
> To... what? Study a civilization and do nothing with the data?
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.
Also, I don't think we can rule out aliens within our solar system. An alien civilization beneath the ocean of some moon?