Isn't there another possibility about intelligent life not having been noticed?
Could it be that they are leaving us alone because they do not recognize us as useful intelligence, in much the same way that we don't recognize an ant colony as a useful intelligence (even though we know that there is a type of communal intelligence, their structures and presence in the world is just an artifact of what we see as primitive rule-following behavior)?
It might also be the case that they have radically different forms of communication that are not amenable to functioning with EM transmission.
Personally, I don't get too wound up about why there hasn't been a discovery of ET. It's just one of those things that will eventually sort itself out, and in the meantime, I would rather worry about making sure that our world is dressed appropriately for any surprise callers (we really don't want our world to answer the Interstellar Door while in our metaphorical underwear, or during a domestic dispute; husband and wife yelling and throwing things while the kids act up in response).
See my response to Mitchell_Porter.
Could it be that they are leaving us alone because they do not recognize us as useful intelligence
But why haven't they already taken all of the matter and energy we're made of? Why is the cosmos in a pristine, isotropic, homogeneous state?
An uplifting message as we enter the new year, quoted from Edge.org:
A few thoughts: when considering the heavy skepticism that the singularity hypothesis receives, it is important to remember that there is a much weaker hypothesis, highlighted here by Tegmark, that still has extremely counter-intuitive implications about our place in spacetime; one might call it the bottleneck hypothesis - the hypothesis that 21st century humanity occupies a pivotal place in the evolution of the universe, simply because we may well be a part of the small space/time window during which it is decided whether earth-originating life will colonize the universe or not.
The bottleneck hypothesis is weaker than the singularity hypothesis - we can be at the bottleneck even if smarter-than-human AI is impossible or extremely impractical, but if smarter-than-human AI is possible and reasonably practical, then we are surely at the bottleneck of the universe. The bottleneck hypothesis is based upon less controversial science than the singularity hypothesis, and is robust to different assumptions about what is feasible in an engineering sense (AI/no AI, ems/no ems, nuclear rockets/generation ships/cryonics advances, etc) so might be accepted by a larger number of people.
Related is Hanson's "Dream Time" idea.