Ive got a 2.0 TDI a4 avant and occaisonally the brake warning light comes on on start up .Not just when its cold but ive noticed this only happens when the turn over speed seems a tad slow thus it might take a couple extra seconds to start.This seems to trigger the warning light and 3 beeps.As said it doesnt happen all the time but i wondered if it might be that the battery is wearing out .