I had the same problem. I spent some time with the manual and on web blogs. I got the
problem fixed and the light is now out! Here is the definitive info:
1 - The tire pressure monitoring system does monitor the spare. This was the
cause of my problem.
2 - Although the spare is mounted under the car, with the valve facing up,
you can get at that valve by using the screw jack elevator system that is in
the manual. You don't have to remove the spare, only lower it enough to
reach the valve which is mounted facing UP. That was easier than I thought,
once I had carefully read the manual and found all the buttons and things to
use.
3 - After everything is done you have to reset the system by using the
button under the dash, to the right of the wheel. You have to be careful to
use the correct button (the left one) so as not to disable the anti-skid
system.
All in all the system is not bad, but Lexus should do a much better job in
its manual explaining things. This could happen several times per year. I
guess they want to have their service Dept. make money.