A civilization that killed itself thousands of years previously might be having the evidence of society preceding its demise (audio/video/etc signals) just now reaching some part of terragen civ, where it can be studied by people who have already seen (from sources closer to the event via wormhole travel) the evidence of that society during its demise (EMPs, metric weapon shockwaves, wormhole collapse energy bursts, neutrino bursts, premature novas, etc) and after (expanding debris clouds, elevated levels of dust-borne radiation, wandering autowars picking off escaping colony ships, populations selling themselves into slavery in exchange for basic lifesupport, etc, followed by decreasing signal sources and evidence of technological regression... ).
It would be interesting to compare the signals generated at different times during the process, and having seen a spectacular explosion at point A, you can retreat ten or fifty light years to point B and set up your array knowing exactly where to point it to pick up the events that led to the destruction.
In fact I can see the lightspeed wavefront of such an event traversing terragen civilization becoming an object of study in itself; radio-archaeologists would know about where it had propagated to and where the listening posts would be in the next couple of centuries, and might build entire careers studying it as it passes by, then relocating and studying it again. So there'd be an expanding wavefront of researchers playing leapfrog with the wavefront of evidence-of-destruction, for many many centuries.
It would be interesting to compare the signals generated at different times during the process, and having seen a spectacular explosion at point A, you can retreat ten or fifty light years to point B and set up your array knowing exactly where to point it to pick up the events that led to the destruction.
In fact I can see the lightspeed wavefront of such an event traversing terragen civilization becoming an object of study in itself; radio-archaeologists would know about where it had propagated to and where the listening posts would be in the next couple of centuries, and might build entire careers studying it as it passes by, then relocating and studying it again. So there'd be an expanding wavefront of researchers playing leapfrog with the wavefront of evidence-of-destruction, for many many centuries.