(Update and resolution, I think, maybe ... )
Brought the car to Big O Tires where the new tires were recently installed. Turned out to be a nail in the driver side rear tire. Fixed. Covered under road hazard insurance. I'll keep an eye on it, obviously, since I check tire pressure at least once a week anyway. For now, my TPMS saga seems to be resolved.
A couple of months ago had new tires installed. TPMS light came on after driving a bit, parking, then starting off again. This pattern continued.
Two days ago, checked the tire pressure and discovered the driver side rear was almost completely deflated. Filled it up, the TPMS light was off, life was good.
Yesterday morning, the light came on shortly after heading out. Checked the driver side rear tire and it was 7 pounds down. Reinflated, drove off, TPMS light went off.
This morning, no light at all.
So ... it seems the TPMS system is working ... drops in pressure trigger the light. But ... how did the tire go almost completely flat overnight? How did it lose 7 pounds the next night? And how is it just fine today after driving, stopping, driving off again ... the kind of thing that's triggered the light before?
I will be taking the car to Big O, the tire shop that installed the tire, and have them check the valve stems, sensors, everything.
I've been dreading having to replace the TPMS system, even looking into aftermarket add-on systems that provide individual tire pressures.
In any case, the TPMS system is 17 years old and if I can just replace old parts with new parts, that is best. Or, perhaps there is just a leak in the driver side rear tire and that's a whole other bit of automotive joy. I kinda hope it is the tire, it's still almost brand new and under warrantee.