More seriously, if we do locate a likely signal, what happens when
thousands of eager listeners buy appropriate receivers, tune them
to that freq, and listen at synchronized times?

The senders, if they are as advanced as we like to imagine, should be able to 
pick up the slight change in energy absorption from our direction, or perhaps 
even pick up the IF oscillation from all those superhet receivers on one freq.

We won't need to respond in any meaningful way; they will know we've
picked up the extension, so to speak.

(Actually this technique is delicate but hardly futuristic; both sides used 
it in WWII to determine if their clandestine signals were being intercepted.)

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