Perhaps we are not looking in the right way
Let’s just assume that we aren’t alone, and that the universe is filled with intelligent life. So far, SETI didn’t track the correct solar system/star to spot them. However, having spend so many ours listening to the sky it seems hard to imagine they just missed them all.
Perhaps other intelligent life forms are simply not sending high power radio waves across the universe for other (perhaps hostile) life forms to detect. But perhaps they are kind enough to send some low energy signal in such a way that other life forms can detect them because all intelligent life forms (at a certain level) are sending the same low energy signal. If there are enough life forms out there this means that we would be able to detect it. Of course no information of location is revealed. Only that there is a pulse of life running through the universe.
How can we detect it?
Not with normal focused radio antennas. We need something big to "see" as much sky as possible.
But what about the Doppler shift? The further away, the more red shift in signal when it is received at our end. So a received frequency curve at earth could already tell something about the distribution of life over the universe. This however implicates that the sent frequency must be known by all senders.
But perhaps there are different ways that such alife signal could be composed such that it can be detected by us.