[ThomasWinwood]

As I understand it the amount of time we were tossing radio into space was quite brief, and more modern radio traffic is directed at the ground (since after all if you're leaking signals into space you're wasting power).

One of the papers cited in the OP argues that a good indicator that there's something interesting about a planet is the combination of molecular oxygen and methane in its atmosphere - the former is a strong oxidiser, while the latter is a reducing gas, so there's some dynamic system producing free methane in the atmosphere.