I think it's not inconceivable that they are alien but not "manned", we've often thought that the distance is far to great to even the nearest possible habitable exo-planet and perhaps it is for humans but not for some robotic machine of extra terrestrial technology. I mean look at all the things we are exploring now in terms of robotics and energy development; a plane that creates its own wind? a robotic worm that projects it's form from within? we've only scratched the surface of what is possible ourselves, we can't just dismiss the possibility that we really aren't alone.