Though in fairness, don't you think they'd have left some kind of probe or satellite; or that they're still watching us from afar? I mean, if they are smart enough to zoom around the universe, they ought to be smart enough to figure out we'd eventually evolve into something significant.
It's a scary door to open, though. Because as easily as the Nazca Lines and a lot of our other mythology could be attributed to aliens, our own existence could as well. Maybe we stepped ahead of the other species on this planet because we had a little help. Who really knows? It's just all really wild speculation. Interesting speculation, though.