The highest power TV broadcast stations could be detected by Arecibo level technology at a distance of a couple of light years. For military radars of the sort used in the ballistic missle early warning system (BMEWS) it's on the order of ten light years, though of course their exact power levels are classified, so one has to make reasonable guesstimates.
If you happened to be in the line-of-sight background of an asteroid being radar mapped by Arecibo you'd get a blast of high power. Though I can't recall distances, Arecibo was basically able to communicate with its twin anywhere in the galaxy; of course you need to know the time, frequency, and location on the celestial sphere to synch something like that up.
Some forty years ago the detection of reflected signals from a few TV broadcast stations off the moon was made.