If there was a huge amount of evidence, it would be unjustified to reject the idea. But there isn't. It's possible to make a convincing case that there is evidence, but when you ask for the solid, unambiguous evidence (i.e. data that would be hard to fake), it's just not there. And you would expect any civilization that is capable of interstellar travel to be able to remain undetected by humans if they wanted to, or if not, you would expect those with the best instruments to detect them first. Mile-across, fast moving objects should show up, not just on military radar, but also on Doppler radar (what they use for looking at rainfall). Fragments from a crash-landed spaceship should have a significantly different isotope ratio than alloys on Earth. For that matter, you would expect a highly advanced civilization to have done some interesting stuff with Carbon (nanotubes, etc), not for them to still be using Aluminum.
Basically, there are a bunch of things we would expect to see if there actually was a highly advanced civilization present and detectable. And we don't see those things.
Recently I've been struck with a belief in Aliens being present on this Earth. It happened after I watched this documenary (and subsequently several others). My feeling of belief is not particular interesting in itself - I could be lunatic or otherwise psychological dysfunctional. What I'm interested in knowing is to what extend other people, who consider themselves rationalists, feel belief in the existence of aliens on this earth, after watching this documentary. Is anyone willing to try and watch it and then report back?
Another question arising in this matter is how to treat evidence of extraordinary things. Should one require 'extraordinary evidence for extraordinary claims'? I somehow feel that this notion is misguided - it discriminates evidence prior to observation. That is not the right time to start discriminating. At most we should ascribe a prior probability of zero and then do some Bayesian updating to get a posterior. Hmm, if no one has seen a black swan and some bayesian thinking person then sees a black swan a) in the distance or b) up front, what will his a posterior probability of the existence of black swans then be?