Currently, SETI looks for intelligent life through radio signals. It turns out that maybe they should be looking at garbage.
An article in the October 19 issue of NEW SCIENTIST speculates that pollution may be a good way to locate advanced civilizations. Radio signals are, on Earth, increasingly being replaced by satellite transmissions and by cable, both of which don't send out the same evidence spaceward. Lighting -- as in a glowing city -- is too faint compared to the light reflected off a planet's atmosphere. But massive pollution leaves clear absorption-line evidence of CFCs, industrial solvents, cleaners, and refrigerants. An alien civilization might not use the same ones we do, or they might get smart enough to ban the really bad ones (as we have CFCs), but on the other hand, they might use something that looks suspicious in the atmospheric composition. We don't have telescopes yet that can do such fine detection work -- but we might soon.
All this means, of course, that an alien civilization can find us through our atmospheric trash. As the Bible says --"By their fruits shall ye know them." It also suggests an interesting SF universe, in which civilizations find and rate each other by their garbage. "Look, Mycbputulr -- chlorofluorocarbons! Oooooo, honey, can we vacation there?"