Aliens have been here, and still are here, don't be fooled by the limits of conventional science. I doubt the believabilty of most contacts, but 'aliens', through methods that we cannot collate with our ultra-immature brains have been here, seen us, study us, and still are here right now, may even be living among us. It is way too arrogant to assume otherwise.
We are just begining to touch technology - macine, organic, and otherwise. Alien races out there have had thousands, sometimes hundres of thousands of years embracing technology of all shape.
Fifty thousand years ago a supernova destroyed a planetary system that had inteligent life on it, developing technologically for maybe 200,000 years. They had to build massive ships - like Noah's arcs - that would send them across the millenia of time to eventaully find a new habitable planet. So on...
I've said it before, these signals, these patterns that SETI is looking for are long gone. They cannot survive these vast multi-million light year expanses in space to get picked up by alien beacons and recievers.
To deny aliens existing is like when they told Columbus the Earth was flat.