The thing about radio-waves is inverse-square-law. I don’t understand why we’d expect our nondirectional radio/TV broadcasts to be distinguishable from background radiation once you get past, say, a light year or two from our solar system.
Earth SETI is basically just looking for directed signals: hypothesised to be either intentional communication attempts directed at us, or signals that we happen to be in line-of-sight of. Omniidirectional signals that could be detectable over interstellar distances would require immense amounts of power at the transmission end, and anyway may even be detected by conventional radio astronomy observations.
The popular conception is that our undirected, historical tv/radio signals might be detected by ETIs - and that therefore this is what our SETI efforts are looking for. In practice, the former is unlikely using known scientific principles, and the latter isn't really being attempted for the same reason.
I don't have a reference for you, but I recall a study a decade or two ago that suggested that we would only be able to detect our strongest radio signals out to a distance of about 10 light years with existing telescopes (I may have that number wrong, but it was not large). That said, with new telescopes, particularly things like the Square Kilometre Array, we have higher sensitivity, and more importantly, we have the angular resolution to measure whether an excess of radio emission is coming from a star or from a planet around that star. So, even a weak signal can be detected, and if we were to find such a signal originating from a planet rather than another object, we could make a pretty good guess that it is artificial in origin.
