About a week ago I started my 1998 Mark VIII at -4 degrees. Just bought the car last August so this was the first time I'd ever stated it at sub-zero temps. The car started fine, but the battery indicator flashed two-four times while I was letting it warm up. The battery and charging system were all checked around the first of the year.
It's possible water got on the serpentine belt and froze causing it to slip. If it happens again, stop the engine right away, then check to see if it is wet. If it's dry, use an inexpensive digital voltmeter to measure battery voltage while the engine is idling. It must be between 13.75 and 14.75 volts. Holler back with your findings.