I got a brake visit workshop message once. About 3 weeks later I got a battery visit workshop message. I had replaced the main battery about a year ago, but I hadn't replaced the auxillary battery. I replaced the auxillary battery and have had no messages since (over a month). If your car is an 03, it may be time to replace the auxillary battery. If you haven't had all the service campaigns, it may be related to the battery control unit.
Since the brake message didn't persist for me, I didn't take it to the dealer. Since your message is persisting, I definitely agree with konigstiger & sosh that you need a dealer diagnosis. You don't want to find out how poor the back up braking systems performance is.