We saw it by accident. It wasn't the sort of extraterrestrial signal we had been looking for, nor the type of star we though it would come from.
Yet it was a clear signal after its own fashion, and even clearer when we combed our observation archives with the help of urgency and hindsight. One of the star's planets -- not, by far, one of the larger ones -- had experienced a frantic rise in atmospheric carbon, followed by a spike of exotic elements not generally found outside laboratories. It was intelligence, if not a particularly smart one. Effective, though. Finer analysis of space-based recordings showed there had been faint electromagnetic signals coming from the system.
The revolution in astrobiology triggered by our discovery was thus shadowed by the socio-philosophical one. But that's not what worries me.
We had never conceived of this kind of weapon until we reverse-engineered these observations. Now we're building them, and I know of only one civilization that did, and they didn't end well. Maybe they'd have been happy to think the light from their demise could be a dangerous lesson but nonetheless one, but I suspect they were the kind of species that would've felt a dark, petty pride in having cursed us to reenact their suicide.
If we do, I hope it ends with us - but I no longer watch the sky at night, or wonder at its quietness.