Rear Vehicle Monitoring system, it was an option on the early sports models - detects a vehicle in the blind spot and flashes a lamp in the mirror if you are about to overtake. I think it works as part of the lane departure warning system. Don't have it on mine, but I do know it makes the mirrors VERY expensive to replace! Can't offer any advice unfortunately, but it should be covered by the warranty, so a call to the dealer would be in order?
Alan