Brilliant - that's exactly the sort of info I was hoping for.
When I start the bike, I get the normal indications (slow flash of Brake Failure, no '!'), and they stop as I ride off. Then, on one occasion 15 minutes into a ride, once on my first brake application, and once about 2 minutes into the ride, I notice the warnings, as described in the title, appear.
I'll check and clean the wheel sensors. However, the fact that the initial test passes, and the slow flashing stops, means the sensors are working at least partially, I guess?
Thanks again for the post.
First of all, to clarify for others reading this, everything posted earlier (and now) assumes you have the IABS with servos (models 2001-2009 with the noisy servos motors when you apply brakes).
During initial internal check phase (after ignition ON) the IABS system can detect if a wire of either wheel speed sensor is broken, HOWEVER it can only detect a weak signal (dirt on sensor -OR- incorrect gap) after riding for a distance. In some cases this fault may appear only at higher speed.
A GS911 readout of faults would confirm if fault is from either wheel speed sensors. As I have posted earlier, the 2 warnings you are seeing could also be caused by an internal fault of the ABS unit, so a GS911 error readout would clarify this ambiguity. IF YOU ARE SEEING THE FAST FLASH pattern (4 times per second) then there is a HIGH probability this an internal ABS unit pressure fault - this is not an easy fix compare to the sensor gap issue. In the USA, no one yet can overhaul these IABS units, but there is one in Germany according to 2 riders from Europe who posted here recently.
Meanwhile, if you are patient and handy, you can measure the air gap for both sensors with bike on center stand. Use a set of feeler gauge to measure distance between sensor and dented wheel - make sure to measure at least 6 places all around as the dented wheel is never perfectly flat. Do not force the feeler gauge in between - it should slide with minimal friction.
As documented in BMW shop manual and CLYMER, acceptable values for air gap are: 0.2 to 1.5mm (0.008 to 0.06 inch)
My experience is that it is much better to be in the middle of this range to avoid a weak signal caused by some dirt on sensors or dented wheel, so I always try to aim for: 0.4 to 1mm (0.015 to 0.04 inch). Now would be a good time to clean any dirt on sensors and dented wheel(s).