I wouldn't waste time looking for signal sources, as the Dynasty expedition beacons in all three zones were POIs that showed up within 1,000 ls distance.
The thing is, earlier logs only mention seeding Earth-like worlds with them, and only the Zurara's notes terraforming candidates too. However, those can be fairly common, so if there were any beacons around them, they would have already been found. Probably with ELWs too, but they are much more rare.
As for the other two megaships: given the distance between the Zurara and the Formidine Rift settlements, if it's a similar distance for the other two, then that would mean a huge amount of systems to cover. After all, those regions have a much higher star density.
Also, could anyone around the Zurara check the distance to the closest system? I wonder if it really was reachable with a stock Cobra or Clipper.