--We may not recognize or even be capable of recognizing an alien signal.
Yeah, that's what I think. I think it's completely possible that within a half century or so we'll have technology that will make speed of light communications obsolete. But of course the physicists poo-poo this and know everthing just like they thought they did around the turn of the last century.
Any space faring race will need a means of communication that's faster than light. Even within our solar system radio communication becomes problematic at distances much beyond the moon.
It is relatively easy to make a signal appear different from a natural signal. We aren't listening for "hello, how are you?" We are looking for anything that stands out distinctly from the natural signals.