As I understand it, SETI is not making any assumptions about what intelligent extraterrestrial life might be — just that, whatever they are, they will be creating some kind on non-random electromagnetic radiation — or at least they were some time in the past, as that radiation is limited by C.

One possibility is that the signal is purposefully hidden in (pseudo) random noise for whatever reason (for instance, current CDMA mobile signal is semi-randomized to make it easier for other users of the bandwidth to filter it out as white noise. Filtering white noise is something we’re pretty good at).

The other possibility is that they are not using electromagnetic radiation for communication. I can think of any number of reasons, but the primary one is the speed limitation of C.

I’m with Fermi: they’re out there somewhere.