École des Bro-ArtsFinally found the gas leak in the basement--or rather, right outside. It's almost undetectable, but diffusing through the wall. Guessing there must be a break in the buried pipe and that's permeating through the soil and back into the house.
Multiple testing crews missed it--doesn't show up in the air or any of the fittings! But every few months I'd go down there and get a whiff of something so faint I couldn't tell if it was gas, something decaying, or I was imagining it.
RopeSwitch@aphyr i’m glad you found it!