We also have to consider the limited frequencies that we have been listening on, even in the EM spectrum. I think that few people realize that SETI listens at the hydrogen line. There has been the Wow Signal, and others, even there. I wonder how loud Earth is at 1420 MHz.
My two thoughts on any alien communications: -They wouldn't use a single frequency, just as we don't use a single band to send robust messages on a wire or over the air. We use a spectrum of frequencies and methods for encoding a signal. In radio we use a wide array of bands for a single signal. In copper, there are ways to encode more than just on off at once, we use multiple wires, and complex encoding schemes. In fiber, there are different colored lights, and different angles you can shoot the light, all which help how much data you can send at once. There is no reason that any beings out there of intelligence wouldn't be doing the same thing. They could be encoding their signals across the entire spectrum of all known frequencies at once for all we know. -Second point, but a bit more out there and speculative. Many scientists have pointed out that if there were advanced species out there in the universe that are say, capable of FTL travel, logic would dictate that they would also probably have FTL communications that we are unaware of and completely incapable of even picking up at this point in our civilization with the tools we have. Unlikely? Maybe. But we've only just begun to start exploring the solar system let alone the galaxy. -You can also encode data at the speed of light using photons, so perhaps we should be looking for visual/optical signals as well. Civilizations could be sending signals with light from planet to planet, but because of the focus most likely needed we would probably need to be in the path of it or looking at a very specific spot to detect this. I believe there is a project out there that wants to do this but can't remember the name of it.
The Wow signal definitely blows my mind. How unbelievable would it have been to be in the room when that was received? I am always tempted by the following two lines of reasoning as well: a) If there were Type 1, 2, 3, or 4 civilizations out there, they could probably choose whether they wanted to be detected or not. b) It could be that sufficient advanced technologies produce patterns and effects that we can't detect with our technologies.
Your lines of reasoning were formalized by Martyn J Fogg in 1987 as the "Interdict Hypothesis." The short version was published in Analog and the long version in Icarus. http://www.sciencedirect.com/science/article/pii/00191035879...
Thanks so much for linking that! I'm going to read it tomorrow.