The chances that they found this planet based on any other fact, other than detecting our radio waves, are non-existant.
If they found Earth before we "conquered" the planet, they would no doubt have taken our pretty blue planet for themselves, wouldn't they?
And if they found it while we were in the early stage of development, why wouldn't they make some contact with us?
So, you're trying to tell me, that they were afraid to make contact with primitave people? Doesn't sound like an advanced alien race to me.
As for the idea that they are "watching" us, and we have no idea how to detect them, is a rather rediculous and conceited theory, don't you think? We aren't that interesting
.