Interesting question. As a previous answer said, based on the scale, it's highly unlikely that we're the only aliens out there.
It could be we're the most advanced, but that's also unlikely. It could be that we're under observation, but no approach will be made until things settle out .. maybe they're waiting for something like all of the polar ice to melt; or until we send a manned vessel beyond our own solar system.
As to whether aliens have been here already .. it's an interesting thought experiment. I think the answer is 'Probably', because, well, wouldn't you want to check out another whole planet?
Alex / talexb / Toronto
Thanks PJ. We owe you so much. Groklaw -- RIP -- 2003 to 2013.