Another possibility is that they've gone through their own singularity, and no longer have a significant visible (to us) presence in physical space (or at least don't use radio waves any more), i.e. they've transcended.
Naturally we can only speculate about what new laws of physics become identifiable post-singularity, but string theory suggests there's a lot of extra dimensions around to which we don't have direct access (yet?). What if civilisations that transcend tend to find 4D spacetime too limiting and stretch out into the wider universe, like a small-town kid making it big?
It look like, if it happens for us, that it will happen within the next hundred years or so. Considering that we've only been using radio for a bit over a hundred years, that gives a roughly two hundred year window for any two pre-singularity civilisations to spot each other before one or both of them transcend. 0.4 seconds in the year of the cosmos.
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.