Earlier quoted context omitted.
There are 100 million times more bacteria in the ocean than there are stars in the universe. Which really should tell people how rare the development of mitochondria was if it has only happened once here.
It didn't necessarily happen only once; maybe we were left with the most successful instance.
1. Died off or regressed
2. Plateaued
3. Is sufficiently far away or stealthy enough for us not to notice them (whether intentionally or not)
I've often thought about the whole communications bubble argument... I don't buy it. I'd imagine, like wifi, other civilizations will maximize their communication bandwidth, which essentially also maximizes entropy which looks like noise to us. Compression, encryption, redundancy, multiplexing over frequency and amplitude and time, directional antennas and signalling... That's what we've done in under 100 years.
I know there's stuff like organic markers etc, but if machines are doing most of the heavy lifting I don't think that would matter. Same with stuff like hydrogen emissions lines. Whatever is abundant will be used for "settled" and "dead" solar systems alike.