Hi Quicksilver.
The alarm will cause some current drain but not much.
I keep my S70 in the garage, unlocked so I don't have that problem.
Mine was caused by accidentally releasing the boot with the remote and not noticing. This caused the boot light to come on and drain the battery over a weekend.
I've now set the light to off so it doesn't come on when the boot is opened.
I appreciate you have an estate but it might be worth checking to make sure you haven't got a bulb lit somewhere.