I think it's +- 4 or 5 PSI change before the TPMS alarms. Tires will lose about 1 PSI for every 10 degree F drop in ambient temperature. If you aired your tires in 70 degree weather then it's possible the alarm would go off at 30 degrees, especially if they were a little on the low side to begin with. Best thing to do is check your cold inflation pressure when the weather turns cold. If you're checking them on a regular basis, then you ought to catch that anyway.