I recall some prominent scientist and cynic mulling over Drake's equation once proposed that each civilization might only broadcast detectable signals for 50 years before succumbing to nuclear war or some other self-inflicted calamity.
Interestingly, I think they turned out to be almost right for a different reason--probably no more than a certain number of years before a civilization would switch to local broadcasts that don't bleed out. So aside from intentional broadcasts intended to be picked up from space, there is a "window" of detectability that further reduces the chance of discovering intelligent life via radio teloscopy. (is that a word?)
I imagine each new intelligent civilization "pinging" that bubble as it emerges, and if it's rare enough all might go unnoticed in a short (less than astronomical) listening timeframe.
"If fighting is sure to result in victory, then you must fight! Sun Tzu said that, and I'd say he knows a little bit more about fighting than you do, pal, because he invented it, and then he perfected it so that no living man could best him in the ring of honor. Then, he used his fight money to buy two of every animal on earth, and then he herded them onto a boat, and then he beat the crap out of every single one. And from that day forward any time a bunch of animals are together in one place it's called a zoo! (Beat) Unless it's a farm!"