Citoen picaso battery empties, 2.0L D 03 plate?

Hi, the battery on the above only lasts 1-2 days when car is not in use especially in cold weather. Battery less than 12mths old, have had to use heavy duty starter many times as battery completely flat. Alternator has been checked, something is using significant power when egine switched off. I have checked obvious like glove box and boot lamps no lights on apart from alarm sensor. I am going to look in to stereo being incorrectly wired and starter motor problem. Car has been on basic diagnostic for power use but nothing found. Any ideas I'd be very grateful. Thanks