Well, think about it this way. Even as studied as our own planet is, every now and then you will here about a new species being found. So if we can study one planet and not find something as simple as a bird or insect. Then couldn't it be possible, that we may simply have gone without notice at this point? At least we are broadcasting radio signals, so that someone out there can see us.
Actually, I think that any intelligent race, would release the benefit of leaving us alone and studying us. Yet you never know, maybe they have controlled us throughout history.