I don't have much hope of finding new Guardian structures, as they seem to have all been found already, but i'm pretty sure there are many undiscovered ruins out there.

Since the answer seems to be that the blue dot doesn't appear unless the system has been discovered, here's how i intend to proceed: Find a nebula that seemingly has no civilization (lots of them around the one where i'm currently at), then criss-crossing it using "economy" jumps, honk every system with the discovery scanner, then look in the system map if it's a high probability one, (namely a complex system with many stars, especially brown dwarfs, with rocky/HMC bodies, 1000-3000km radius, 180-310K temperature). If not, jump to the next system. If yes, "sweep" it for any mention of ancient ruins (ruins are visible from far away as long as you don't select any nearby target).

Rinse, repeat, until i get bored/frustrated and resume my trip to Colonia and Beagle Point.