Flat Battery - How best to avoid ?

[Muz]

I last drove my E70 last weekend for several hours everything normal,.. went to use it this weekend, and the battery was down in power and the car wouldnt start. Electrics came and went , but nothing when I pushed the button. The car was in my garage, unlocked and all doors shut all week.

This I thought, was the best way to leave it when not in use, without the alarm being activated, as my experience with my last E53 proved, which suffered a simial problem but it hasnt worked for this E70, something must have been draining power.

I did leave the lights set to auto, but I hardly think that is the cause. Aside from pulling the battery out, charging it, then doing a heavy discharge test on it to see if its retaining power, any ideas/known issues ?