after you confirm your gauge's accuracy, how about a little experiment to see if one of your 4 sensors is bad. It might take a few days to work the experiment, but if you can trouble shoot th problem, a trip to the dealer might go faster.
set the front at 31 and rear at 38 and see what happens. if no light, then drop the fronts to 29 leaving the rear at 38. still no light, then set rear to 36. if you can narrow between inaccuracies front and back, then you MIGHT try varying pressure between right & left. you probably get the idea
I KNOW THIS WILL AFFECT HANDLING AND MIGHT CONSISTUTE A SAFTEY ISSUE. IT IS ONLY A SUGGESTION AND WHILE DOING IT, BE COGNIZANT OF THE HANDLING.
heck you can try it while the car is parked.