I noticed that the tire monitoring system seem to notify me about the left rear tire in my 03 A8L. This is despite checking the tire pressure which is okay and the same as the rest. I usually put 40-42 on all tires. Could there be something else I am missing. The car is still when I ask the system to remember the current pressures.

Is the spare tire okay as well? It does count that one as well. (If you add another tire with TPM in the trunk it acts weird f.i.)

If that's okay, it may be the valve or the antenna playing up. If you don't have a VAG-COM, try swapping the wheel, see if the low reading shifts... If so, it's likely to be the valve, if not, it may be the antenna.