The Bob Dickman/Les Schwab in Junction City, OR sold and installed my Michelin tires a few years ago. I think it was about the time of the Bac to the Future rally. I had the factory flexible hoses replaced with the solid extensions and wheel grommets. Last year I developed a slow leak of about 5 pounds a week. I went to a Les Schwab in Milton (near Tacoma) and asked them to find the leak. I thought they would find a nail. They took the wheels off, sprayed the soapy water and found the exterior valve stem nut needed about a quarter turn. They put the wheels back on and charged me nothing. This year exactly the same thing happened on the left side. Again no charge. The Les Schwab chain is a nice place to do business.
Apparently with around 30,000 miles of driving, there is enough movement of the stem stretching between the wheels that vibration will loosen the valve stem nut. Either that or the first installer didn't get it right.