> 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).