Had a few flat batteries recently and just got to the bottom of it. it seems my glovebox piston was snapped - quite common. Although I didnt even know it was supposed to slowly drop down so never noticed it. It seems that if your leave the interior lights on normal, the car dims all the lights regardless of if the glovebox/boot etc are left open. But! if your turn the interior light off, your glovebox light doesnt go out! So because my glovebox was broken, I actually caused the battery to go flat by flicking the interior lights to off!