Fermi's Paradox has so many holes its untrue.
1) It assumes that ETI would be technological, look at the distribution of technology on the earth... Not everybody can build a radio.
2) It assumes the EM spectrum is used for broadcasting (not a bad assumption at face value). Also that technological races have to have communications of some sort, to go for true pulp sci-fi telepaths don't use radios lensman. However we don't know what else is out there. Also we don't know how far away this civilization would be, perhaps their signals have long since been drowned out by other signals from stars, quasars etc...Or they've discovered a way of infering a shear signal onto background microwave radiation, allowing them to communicate FTL. Unless we knew what we were looking for and where to look we could not intercept that signal.
3) It assumes we could recognize such emissions if we saw them. This is an ETI we're talking about, how would we recognize EM broadcast Pheromone transmissions? How would we recognize mathematics based on a non-linear number system, or something uninvisioned on earth.
We tend to think in terms of sound and vision, imagine a race for whom these were entirely new concepts, thought patterns would be radically different, communication of any sort might be utterly impossible between our peoples unless we could find some common ground. Even mathematics is based on our perceptions of the universe around us. It is not a truely universal language, we can just hope that it is pretty broad.
4) Its assumes that ETI would want to contact us (not sure I would). With time our own systems will probably become lossless as we don't want to waste resources broadcasting to the universe in general.
5) It automatically assumes Von Daniken and his cohorts are wrong. I certainly wouldn't discount some alien race having visited the earth a long time ago. Pick up a few choice specimens and then on to the next locale. If they wiped the dinosaurs out we'd never know now, what evidence would survive 65 million years- which lets face it is a long time for a mountain to last let alone a species or civilization.
And that's without getting into the idea that a race would expand spherically throughout the universe (we percieve it as the best way of expanding a higher order intelligence may disagree as the expansion front must become thinner until possibly the expansion front is so weakened that it can't manage to secure a resource to replicte the von neuman machines). Even on earth few animals expand in a circular manner from an initial point. Look how much trouble we've had tracking mans own history, people tend to jump from location to location almost at random, you might spot a trend over a thousand years, but on a yearly basis its pure mayhem.
Or that we could be isolated from the rest of the Galaxy by higher technology races deliberately. - interstellar conspiracy theories anyone.
Its also assuming that one of the many mystery objects that astronomers have looked at a come up with some startling theories regarding (quasars, pulsars et al), aren't infact observences of huge generational ships travelling through (or away from) the galaxy. Remember there is no real evidence either way, nobody has been to have a look, we just have speculation based on size and power of emissions. -Although I wouldn't want my old physics professors to hear me saying that
The only way we can ever really know the answers is if we manage to get out there and have a look. Unfortunately unless science can crack wormhole/warp/bloater drive or some other FTL theory we'll be guessing a long time.