You return today and see that one previously unremarkable ape-like species has spread out to all parts of the world and is even orbiting in little tin cans around the planet. Almost all wild terrestrial megafauna is gone and the megafauna that remains is raised by the "humans" in massive farms. They've converted forests and jungles into farms and cities, and they're spending vast amounts of energy to make silicon think.
The fact that aliens have not nuked us from orbit out of sheer panic is, I think, strong evidence that we are alone in the galaxy.
