I'm no mechanic but I am a reasonable electrician. I have suffered the exact same problem with my 08 S-Max after the car was left for a few days or I left the fridge on whilst camping. The camping incident was the first to show the issue as I thought I'd simply ran the battery flat but the car was not just low on electrical power, it was dead. 2 hours later the AA turned up but after 45 mins he was scratching his head calling for recovery. I suggested something that seemed outlandish but the AA man said why not as everything else had failed. What we did was to plug a cigarette lighter plug into the cigarette socket and dabbed the positive wire onto the battery directly. The result was that it "woke up" the car. It was today that the second occurrence happened and I tried the trick again and again it worked. This time the car had been left for 3 days with no obvious drain on the battery. One of the things noted was that there was no power available at the cigarette socket or the interior light but there was AFTER I simply back fed the cigarette socket by dabbing the positive terminal on the batter to the centre pin on the socket. I now carry a lead round with me in case it happens again. Interestingly and unsure if its related. I recently had a ABS brake issue and Ford chose to replace two looms that they deduced as faulty. Hope this helps and do the above at your own risk.