Well, while the ruins are much larger than geyser / fumarole sites, they don't show up as blue circles on the radar. So the only instrument you can use to find them are Mk. I Eyeballs. Depending on visibility, they can be hard to notice even if you do know their coordinates - or maybe it was just me back then, heh. It would be nice if FD placed these ones on tidally locked bodies though.
My theory is the ruins are still powered because they draw power from geothermal sources, so look for volcanism and then check the area for ruins, that's my plan anyway!