Good point. Brian Cox pointed out in the aforementioned podcast that their space probes could be as small as iPhones with a similar energy output. They could be in orbit around earth and we wouldn't be able to detect them.
I do still feel that, most probably, the distances involved are the bigger factor, though. Even if we assume that every single civilisation sooner or later gets to a point where they emit radio waves (which we’ve been actively looking for for a couple of decades now), then those only travel at the speed of light and might take tens or hundereds of thousands of years to get to us. And that is also assuming that we actually would detect them. I feel like radio waves from a civilisation could easily get lost in all the other stuff out there that’s emitting similar radiation. Especially when we’re talking the half of the galaxy we can’t really look at well. I think in some respects it would probably be easier to monitor other galaxies than it would be to figure out what’s going on in parts of the Scutum-Centaurus Arm.
As far as Dyson spheres are concerned, I always thought that was a stupid theory. I have a feeling what we are seeing there is a natural phenomenon we can’t explain. Probably to do with dark matter or dark energy (it makes up 95% of the stuff out there, after all). I have always felt that if you were sufficiently advanced to build a Dyson sphere, you’d easily find smaller, more subtler and — most importantly — more efficient ways of generating energy. It’s like Ian M. Banks and his Culture orbitals … why not just colonise a few planets and save yourself all that work? Even if you have AI and robots to do it, it’s just tremendously wasteful and inefficient to build things that look like planets in space when you might as well use the actual planets that are there. These things feel like too much like ’70s sci-fi ideas inspired by utopianism / communism to me. They’re neat ideas, but I am not convinced anyone would actually go through the trouble instead of just building something smaller and more elegant. Looking at actual history, humans never actually behave that way. And since I have no other reference point, I must assume that NHIs would behave similarly.
Love the snowflakes on your blog, BTW. That made me laugh. 
Fabian A. Scherschel
