I had the very same problem...for the life of me I could not locate the bulb as every single light on the car worked. When i took my car in for an MOT the guy told me he replaced a 'brake' light bulb because he saw the warning light on. The reason why all the lights were working was that the brake light bulb filement was broken but still making contact with the prongs inside the bulb..but any judder and it would lose contact. I checked all the bulbs but not close enough. If this happens again in future I know what to look for. Just pull the brake light bulbs and check them out very carefully as I am pretty sure that's your problem.