It turns out that the leak under the master bath sink was not the fault of the faucet, but caused by a modification that had good intentions. Here's the story: Our Rheem Tankless Water Heater is designed to not fire and heat water unless the flow is substantial enough. The special ultra-low-flow faucet aerators on the 2 bathroom faucets did not allow enough water to flow, so we could never get hot water in either location. So just last week, I replaced the low-flow aerator with a standard one, and since then we were enjoying hot water at the master bath sink. But for some reason, the new aerator didn't fit quite right, and it was allowing some water to trickle back into the faucet cavity, which then found its way out and into the enclosed vanity below. After thinking about the issue since last night, I decided to investigate and I'm glad I did. Not only would I have never known the true cause of the problem, but I would have wrongly accused the makers of American Standard faucets of manufacturing faulty goods (I will go back and edit the previous post where I made that accusation).
Today I learned: Water leaks are difficult to track down.