Home built 1938.
Galvanized plumbing vents go up the exterior walls, make a 90 degree turn, then another 90 and exit the flat roof, pretty typical design.
The problem turns out a bit subtle. The vents are open up top, and rainwater and dew is expected to enter and drain into the sewer. But if you imagine a galvanized elbow in cutaway (I can’t post images), you’ll see it always has a little shelf the depth of the connecting pipe wall. It’s inherent in the design.
Thus, for 77 years a little bit of water sat in each elbow, until maybe it dried out in summer. That elbow got thinner every year until finally… a slow drip perfect for attracting termites but not a big enough leak to call attention to itself.
As for what to do about it, I open that to the forum.
Keywords: old house plumbing DWV drain waste valve leaks inspection.