My car does this from time to time. Not sure what year your car is, but on my 97 the system doesn't really read the air pressure. It reads the differences in wheel speed. So I notice that the light will go off if I am driving slow and hit a bump/pothole. Or it could be a wheel speed sensor starting to go bad. And sometimes it goes off for no noticable reason.
But when it does, I just reach under the dash and hit the reset button. If I really have a tire going flat, I just figure the light will come back on shortly.