Alright, I took the tube out and was ready to try the under-water test, but did the lip/face test and finally found a very small puncture. As I hadn't kept track of where the tube was on the tire initially, I spent some time examining both the rim and the tire. I found a tiny little rock-thorn thing sticking through the tire. It didn't seem to be protruding into the tire when I found it (only outside of the tire), but the puncture-to-valve distance seems to match, and it certainly was sharp, and I imagine with 130 PSI and 100-200 pounds of load on it would allow for plenty of opportunity for puncture.