[Water Warrior 2]

Another far fetched theory. When the shop installed the new tire etc did the tech tighten up the nut holding the valve stem in place. If it is too tight it will damage the base of the stem where it comes through the rim and quite possibly create a very slow leak. In my zeal to tighten stuff I did this years ago and had to buy a new tube. The shop guys said "Finger Tight" now give us your money. I think I deserved that one.