Honestly, if aliens have advanced enough technology to take their gigantic death-dealing starships the millions of lightyears needed to find Earth and strip it of resources, I suspect that they wouldn't need mere radio signals to find us - they could probably identify Earth and all its resources via a simple light spectrum analysis, which travels just as fast.
However, given the infinite possibilities the universe has to offer, I'm inclined to believe that they already know about us. We've more than enough people claiming to have been abducted by them. I'm not sure I'd go so far as "walking among us," but it seems to me that humanity is very much being monitored and/or manipulated.
It makes a certain sense, really, if you think about it. Lets say you have a space ship capable of transversing the cosmos. You discover this planet where intelligent life seems to be evolving. Do you leave them to be, risking the chance they'll turn into something that comes back and threatens your race later, or do you intervene and try to shepherd them into something that isn't a threat? I imagine this would be matter of some debate, but the thing is, the means are available, and at least one alien race out there is likely to have made the latter decision.
Given that it takes millions of years to evolve, if they wanted our resources, they'd probably have had them by now. Yet, again, when you have the means to transverse the entire universe, mere elemental resources really aren't that rare.