Indeed, more and more of the Earth's electromagnetic radiation will be designed to be indistinguishable from noise. Or it will propagate in optical fibers. Based on our own experience, I give a civilization a roughly 100 year period during which their radio emanations will be intelligible. A planet interconnected by optical fibers is kind of a poor man's dyson sphere. ;-)
Yes. Some years ago I went to a talk on SETI at Stanford, where they talked about detecting "carriers". Anything they could detect is obsolete technology now. Analog TV was 80% carrier (video was AM, the audio part was FM) with a huge sine wave component. Digital TV looks like narrowband noise.
Analog TV was probably Earth's biggest detectable RF export. Lots of transmitters in the megawatt range transmitting 24/7.
I think technological progress in modulation might be a really good explanation for the "great silence." Advanced aliens could be all over the place, but if they're really advanced their transmissions are low-power, directional or cellular, and look like noise if you don't know the codec.
I thought I remembered reading somewhere that even our most powerful emissions were unlikely to be detectable to any type of receiver we could conceive of further than a light-year or two, less than half of the distance to the nearest star.
