But the SETI assumption is that an advanced civilization could beam powerful radio signals into space to let other civilizations know they're around. That's par

But the SETI assumption is that an advanced civilization could beam powerful radio signals into space to let other civilizations know they're around. That's part of the premise for Sagan's Contact, with the aliens listening in and then sending a strong signal upon detecting a new fledgling civilization. From the movie, it's apparent there were various listening posts scattered about the galaxy.