I really think that if we ever actually find evidence of extraterrestrial life or civilizations, it will be a result of spectrographic readings of the atmosphere of a planet around another star. Any alien with half a brain looking at the spectrum of our atmosphere will see the unmistakable signs of directed, "artificial" activity. Not just traces, either. It's obvious. Maybe someday we'll see the same thing.

The problem with searching for traces of alien activity on our own planet is one of differentiating it from our own activity. We'll be looking for aliens who are kind of like us, and as a result, their traces are naturally going to look like things we do or have done.

Unless they made really obvious signs, which they obviously haven't.

And then, at the end of the day, we're still going to be stuck with the problem of differentiating the activity of real aliens with the activity of our own human time travelers fiddling around with the past (or future, or alternate timelines, or whatever, depending how you look at it). Yeah, they're technically "alien." But, come on...