there are two different types of sensors...
one was described by the guy above me where the vehicle uses the WSS (wheels speed sensors) and calculates an average to estimate approximate air pressure based on a difference of wheels speed between the wheels.
the other type uses actual pressure sensors in the wheels to give a real time actual air pressure. this can be displayed in the DIC (driver information center), however the air pressure light will only come on if there is a pressure difference of 8 psi between wheels, so if all of your tires lose 10 psi over the period of a week or a month or whatever, the light will never come on, only if there is a difference of 8 psi.
this system, also requires that if you rotate tires you need to recalibrate the sensors using a GM specific tool to do it... on some years and models you can set up the programming feature by a specific procedure of turning on and off the lights and opening/closing the doors and then release a couple of psi of air from each tire, one at a time to set this. you will have to look it up to be specific.
GM tends to use the system I bolded, so if you checked all your actual pressures and everything is ok, then you either have a wheel that is not in it's proper location (recalibrate your sensors) or a malfunctioning TPMS sensor.