Was the bike on the car rack in a public area? Maybe some kid thought it'd be funny to let the air out and make you wonder.

Though there are unexplained times where sudden temperature changes leaves one of the tires on my car or truck flat and the others seemingly okay with the temp change. Its rare, but it happens. Just put it out as a Murphy Rule thing. If it hadn't been that, then it'd have been something else that got you.