It was a joint failure, basically the temperature extremes here caused stress fractures from continual expansion and contraction.
This is not a sewer line, it is a septic-system fume vent that was leaking into the house. And yes, thank you, it is all sorted out now.
And also thank you for the information about ABS, UV inhibitor, and the durability of same. I'll stop planning to have all the ABS replaced. ;-)