First, never before in history will the sky have been searched so thoroughly (for example, by Ohio State, the Harvard/Planetary Society, NASA and the Soviet SETI projects). Also, the Planetary Society is now discussing with Argentina the possibility of establishing a Southern Hemisphere SETI.
Second, an advanced civilization may use a sizable fraction of its sun's energy output. This may lead to detectable side effects, just as our radio, radar, TV and atmospheric nuclear weapons tests have produced inadvertent signals an alien astronomer might find.
Third, the numerous Earth- and space-based observatories could find the first signs of another civilization in some puzzling astronomical phenomenon.
Fourth, a communications engineer could detect a celestial noise source that turns out to be from another civilization.
Fifth, amateur SETI projects in the United States and Canada might find a signal on a frequency the professionals have dismissed for incorrect reasons.
Sixth, military electronic eavesdroppers could discover a signal more alien than they bargained for.
Seventh, we may have no idea of the best way to communicate across interstellar distances. If so, neutrino or gravity-wave telescopes, or even giant particle accelerators might find an unexpected artificial signal.
Eighth, the interstellar garbage of a civilization might well drift through the galaxy endlessly. There must be orbits associated with planets such as Jupiter that capture interstellar debris preferentially. Then, too, there are asteroids that move at about the same speed as the local interstellar neighborhood, where relatively gentle collisions may allow some interstellar debris to survive impact without vaporizing. Deep-space missions may one day find some piece of debris, even if only microscopic, that could not have been produced naturally.
Finally, perhaps the evidence already sits unrecognized in someone's laboratory.
Smith may be right. But with these diverse channels now opening on the universe, it seems to me that if other civilizations have arisen within our galaxy, the chances of detecting them within our lifetimes are excellent.