In this Closer to Truth interview with computer scientist and futurist Jaron Lanier, he's asked why we haven't detected other signs of intelligent life in the universe. Jaron does a decent job of explaining some of the difficulties of looking in exactly the right place at exactly the right time, and then offers an insane proposal.
For any advanced civilization that wanted to signal their presence to the universe, they would likely want to leave something that could be detectable on a long timescale from a great distance. He suggests that such an advanced civilization could do something like move asteroids or other heavenly bodies into configurations that couldn't be natural and could be seen from far away for millions of years. He suggests that maybe we should be looking for signs of such cosmic bat signals.
In the video, Jaron and host Robert Kuhn mention and snicker over what's been called the Happiness Box theory that attempts an answer to the Fermi Paradox. The Happy Box theory posits that sufficiently advanced civilizations don't swarm out into the universe, they implode into virtual worlds of their own making. If you could gain the ability to upload consciousness into simulated realities that are functionally indistinguishable from physical reality, why would you bother to climb into a physical spaceship to outwardly explore? I've always found this idea a compelling possibility and wouldn't laugh it off so easily.
Bonus video: SETI founder and astrophysicist, Frank Drake, is asked the same question but gives a very different answer. He says we haven't found signs because we haven't really looked. We have devoted mere needle in a haystack levels of resources and commitment into what is an impossibly vast search.
And then, there's always the Dark Forest theory…