One of my tires was driving me crazy. It would only lose pressure when the temperatures got cold outside. Parked in the garage it barely lost a pound a week.
So I had the mechanic look at it. In the warm garage, the bubble chamber showed no leaks. We said its gotta be on the rim.
sure enough. The alloy wheel had corrosion all around, it look like salt scum. They cleaned it and re-seated the tire and no more leak.
Considering how corroded it was, I'm pleased that the tire had enough flex to not leak most of the time. I have Coopers on right now.