I think you're on to something, Tom, about the short time a civ remains radio-noisy. Given that, I wonder about the danger of saying 'hi' to the neighbors.

One way of seeing it: A civ that can do the exotic physics we wish were possible -- that is, one which could get here from there -- has either dumped the RF spectrum for something as exotic as their stardrives and pays no attention any more, or they're already on their way (if they were already here, and hostile, we'd know).

Another way: A civ that's still as focused on RF as we are is likely to be no more advanced than we are, and can reach us no more easily that we, them.