Update: I took the tire back to the dealer a week ago. They said they found a previously installed plug that was leaking, and they installed a better patch. Two interesting things about that. First, why didn’t they find that the first time around? Second, they are the ones who installed that plug originally. I mentioned that, but didn’t belabor it. Sounds like the quality of my repairs depends on the tech doing it, phase of the moon, day of the week, whatever...
Anyway, it’s held pressure for a week now, and they didn’t even think of charging me.
I’ve been dealing for a few weeks now with the case of the missing tire pressure. I’ve been losing about 10 pounds a week on average, but it’s been accelerating over that time. This pic was taken on Sunday. It was 30 pounds on Friday.
I took it to my local tire shop and then to the dealer who originally sold it to me. No one can find a puncture or a leak, not in valve tread or sidewall. The only thing I can think of is a deformity at the bead or somewhere that only manifests when the weight of the car is on it. It must be the tire or the rim, and I fear I will have to buy a new tire to test which.