The issue is not radio waves--it's Dyson spheres (look it up). It does not take very long at all for a space-faring civilization to advance to such a point that it's artifacts would be (spectrographically) visible from our telescopes today.
It doesn't have to be Dyson spheres either. A large object moving near the speed of light through the interstellar medium would give off one of a few very specific and very strong spectral lines. Now there hasn't been a concerted effort to look for interstellar travelers, but nevertheless in all our years of searching such oddities haven't shown up.
Our nearest other star is 4 light years away. 1000 light years away may still not cover many planets good for ET to colonize. But for us now to see evidence of a colony on a planet 1000 light years away or more would be TOUGH.
Note: The planet hunters mostly don't actually see the planets but just shadows or evidence in star wobbles, etc.
But we are putting up some new telescopes with some astounding resolution, etc.
It doesn't have to be Dyson spheres either. A large object moving near the speed of light through the interstellar medium would give off one of a few very specific and very strong spectral lines. Now there hasn't been a concerted effort to look for interstellar travelers, but nevertheless in all our years of searching such oddities haven't shown up.