If we found evidence of Aliens, we should try to be really silent and limit the amount of radionoise we emit.
Most likely first contact scenarios: A. They're good and tech-superior; thus, we have much to gain from them B. They're bad yet tech-inferior; thus, we have nothing to fear C. They're bad and tech-superior; and thus, we are doomed D. They're good but tech-inferior; ergo, they have much to fear.
First, we will ascertain their intent and capabilities before risking exposure. Assumptions are inappropriate in uncharted scenarios like this. Caution and preparation come first, followed by a warm welcome if they turn out to be friendly. We can learn a lot from each other if given a chance.
If they are technologically advanced enough to be a threat to mankind (i.e. travelling long distances and having advanced weaponry), then they must be able to discover our planet without the need of us sending them signals.
In space, no one can hear you scream... Seriously though, the genie is already out of the bottle. We have radio and television transmissions already that will reach "them" before long. Any new ones would not get to them any faster.