When I start my '52 plate Audi A4 convertible 2.5 tdi, the brake warning symbol and buzzer show on my dash for a couple of seconds but then go out. This only happens if the car has sat for more than 5 hours without being started, or I have only done a short trip and the engine isn't very warm, and then start the engine again. I have taken it to be looked at by the garage, but there are no fault codes stored, and upon inspection, there is nothing wrong with the brakes. Does anyone have any ideas as to what could cause this? Surely if there was a problem, it would show on the dash for more than just a couple of seconds after the engine was started? I' confused and don't know where to turn next.