[goodcanadian]

Thus, the fact that we are unable to detect any signal from even a single other civilization is quite puzzling, we should be inundated by those signals.

People overestimate the detectability of our own signals. I don't have a reference, but I read an article a few years back where someone calculated how far away we would be able to detect our own signals with our current technology. The answer was about 20 light years. There are only around 100 stars in that sample. About all we can say is that there is not a civilisation similar to ours in that sample. Anything further away would have to be deliberately trying to communicate with us. If they are similar to us, they wouldn't know we are here. If they are much more advanced, we might be uninteresting to them for any number of reasons.

Technology advances, and this situation will change. The Square Kilometre Array, for example, will give us the resolving power to see whether excess radio "noise" is coming from a planet rather than a star (which might be an indication of an intelligent origin). This will increase our bubble of detectability much further out. Unfortunately, I don't have a number on that, but it is still an incredibly small volume compared to the size of the universe.