If they were here, would we know?
If we were to detect, say, a century from now (to be pessimistic about the advance of our technology), another civilization which we were interested in visiting, I suspect that we would have many technologies at our disposal for doing so undetected. Nanotechnology is the most obvious.
Given this, I find it impossible to believe that any visitor to our civilization would be so clumsy as to attempt and fail to conceal themselves, as so often happens in popular accounts of extraterrestrials.
It is an unpleasant thought, but the possibility that we are being watched without our knowledge does not even require invocation of Clarke's Third Law any more. Of course, the probability of such surveillance is another matter, but set that aside for a moment.
A better argument against extraterrestrial intelligent life is that we haven't heard them yet...the "Why is the universe silent?" question. As the SETI folks argue, it makes more sense for us to listen. Nothing on the radio yet, though. Perhaps we should be theorizing about how our technology may soon advance to the point where radio waves will no longer escape our planet.