It's a fairly common feature on newer vehicles here, at least the ones with anti-lock braking. I believe the low tire pressure is actually estimated based on the difference in rotational speed between two wheels on the same axle, using the same sensor data that operates the ABS. The vehicle's management computer monitors that data and when it detects enough difference to be significant over a long enough distance with no braking or turns, it activates the warning light.

This is conjecture in part, but the behavior of my car's system (it's a 2007 model) is consistent with that explanation. If the air leaks out while the car is not in motion, it takes perhaps 500 feet on the road before the light comes on. When you top up the air, the light stays on until you've driven about 500 feet or so at a steady speed. During a period a few years back when the slightly undersized spare tire was on one wheel, the low pressure warning stayed on continuously until the normal tire was repaired and replaced.