OK, I'm an idiot.
GF came by the other day and said her driver-front tire was low (pressure sensors in car told her this). So I put SOME air in, and then checked it, and it was OK. Sensor still said low. So she already had an appointment to have it checked, and I said they can check the sensor.
Now, what REALLY happened, is that in her last oil change, she had the tires rotated, but they didn't update the sensor locations. So....it was the passenger-rear tire that was low. I should have checked all the tires, but I didn't know she had the tires rotated recently.
Anyway, so she took it in yesterday AND THEY DIDN'T TAKE THE TIRE OFF to test for leaks. Grrrr. So she'll have to go back again if it's loosing air (slooooow leak though). But they did reset the sensor locations.
And in my own defense, I've not had a car with tire-pressure sensors on it, that's a new thing these days, and I've been care-giving at low $$$$ trying to get by, so they've had them since 2008 model year I guess.
Duh.