Well, after an hour-and-a-half of digging four different holes in my yard, my neighbor and I tracked down the leak in my main line. Once again, it was a joint coupling that cracked. This is the second crack I've had at a connecting juncture, the first was a L coupling close to where the main line meets the house.
The pressurized spray leaked a surprisingly high volume of water. The break itself was hard to pinpoint given that the water seepage run all the way along my main line, from one end of the yard to the other. As a result, we stomped around until we sunk into a particularly squishy section of yard and dug till we found the main line and the tell-tell sign, bubbling water. It was almost blind luck that we found it at all. But the fourth hole yielded pay dirt and cutting and refitting a new section of pipe took maybe ten minutes.
I'm averaging a break in my line every four years, so I pray my neighbor doesn't move away.