[giantrobot]

The only signal we could possibly detect would be a deliberate one. No one sending a deliberate signal to an unknown alien civilization is going to encrypt or encode everything. Besides being unintelligible they'd literally look like noise and be discarded by the receiver.

It'd would make more sense to use a beacon with some obviously non-natural pattern. Then something like the Arecibo Message[0] on some mathematically related frequency.

The likelihood of picking up spurious stray signals from an alien civilization is vanishing small. Even our most powerful broadcasts from Earth will fade to noise before hitting Pluto. Radar (weather or military) might be detectable half way to Alpha Centauri. You'd need a very narrow beam (to focus the input power) to get a detectable signal a few light years distant.

[0] https://en.m.wikipedia.org/wiki/Arecibo_message