Michio Kaku talks about what would happen if SETI did discover unambiguous alien communications.
Personally, I think it's moot because as I've stated before, the SETI protocol for confirmation makes discovery of alien communications
To recant; SETI first listens to very narrow band channels, thus it is unable to detect modulation or sidebands, it is only capable of detecting
carriers. Most of our modern communications is moving away from simple modulated carriers into modulation schemes that spread the information out
across a very broad bandwidth because these schemes make the most efficient use of bandwidth, power, and are most immune to interference.
SETI would not be able to receive any information transmitted and with our most modern communications it wouldn't register anything at all except
perhaps a slight rise in background noise.
But in addition to that problem, even with huge amounts of power, with the distances involved, a signal could only be received with very high gain
(dish) antennas pointed directly at each other.
Because the Earth is rotating, and orbiting a star that is orbiting in a galaxy, and the galaxy itself has motion relative to other galaxies, and
because the distant transmission would likely also be on an planet that is rotating, orbiting a star, which is orbiting in a galaxy that has motion
relative to our own, any chance alignment of two high gain antennas would be brief.
But SETI protocol is as follows:
1) A candidate signal is received.
2) The antenna is pointed off a few degrees to confirm that the signal is indeed from space and not terrestrial interference. If the signal goes away
then the antenna is pointed back to the original location and the signal is looked for again. The beam width of the antennas is probably sufficient to
allow this step to be completed.
3) Another site distant on the Earth is notified to look for the signal. This serves to eliminate sources near Earth from Earth orbiting satellites.
Although a catalog of known signals is used to first eliminate signals originating from satellites, the military does not publish all of the signals
it's satellites generate and therefore this process is necessary. This will necessarily take some time and there is a strong likelihood that the
antennas will no longer be mutually aligned by the time it is completed.
4) Assuming by some miracle that step 3 is completed; the next thing that SETI does is wait a day and then look in the same place. By now surely any
chance alignment that existed will be gone and no signal will be found.
The ONLY way that SETI will detect an alien signal is if they crash a saucer into the building or aliens are intentionally beaming a signal
specifically at us and keep re-aligning their antenna to track the movement of Earth relative to their position.
Given all of these variables, I don't think SETI is ever likely to detect anything making what they do after detection entirely moot.
That said; SETI has detected some signals that appear to be extraterrestrial and not possessing characteristics of any known natural objects, but none
of them were observable the next day thus none passed SETI protocol requirements.
There are many natural sources of radio waves, such as hydrogen gas MASARs (a microwave analog of a LASER), that, with a receiver only capable of
listening to an extremely narrow bandwidth, would be very difficult to discerne from any artificial source.