Chronic flat tire, no one can tell me why?

OK I'm running 18" alloy wheels and I just moved to Idaho where its been around 5 degrees overnight. So this one tire decides after a week or so, its gonna dump all its air overnight. I take it to Firestone, they can't find anything wrong. I figure OK maybe it was a freak incident. Wrong. I leave it overnight again bam same thing happens. I take it to a different tire shop, they can't find anything wrong. They put it underwater...no bubbles at all. That same night it dropped from 40psi to maybe 10. And the other tires are fine. Should I blame my tpms sensor?