Are you certain that the warning isn't coming from one of the tire pressure monitors? I see that the warning is displayed on the instrument cluster...and based on your model year...you have the type that send a signal to the DME. If all 4 tires that are mounted are okay...remember that the spare also has a sensor...and if you haven't checked the spare...it may be low.
Does your owners manual mention anything about the tire pressure system giving an audible warning as well as the written warning that appears in the cluster? Let us know if it does because there is a change in production as far as the tire monitoring systems goes. The latter e53 (like my 06) uses a different set-up...which uses the wheels sensors to detect a difference in rotation to determine is a tire has a different inflation than the other tires.
You can see in the chart below that 10/04 build date is where the tire monitoring system changes for the e53. Again, the earlier system uses yellow tell tale (light) + text + gong to alert the driver...whereas the latter only uses the yellow tell tale:
