Interesting to see how long this post has endured, clearly an evergreen problem, So I thought I'd add my experience.
The first I knew of a flat battery was when my neighbour - nearer to my charging spot - called to say my windscreen wipers were doing some strange little Tango. When I went to investigate, they were moving VERY slowly over windows. But they were not turned on - and I couldn't turn them off or stop them. Maybe that's actually a preprogrammed visual sign of a flat battery, I kept pushing held down the on/off switch and they eventually stopped their dance.
It was the day after I'd got the car and I think I left some things on while playing with it.
Fortunately, the car was unlocked but still couldn't raise the tailgate - had to do the boot release thing, with a screwdriver slide over the little white lug under the removable cover on the inside of the tailgate - centre near floor level.
Removed the floor section in the boot, 5 screws, the battery showed only about 6 volts. Later discovered you can get the same reading from the + post in the fuse box, under the bonnet.
AA came the next day, put the little Jump Booster on the + terminal in the fuse box, and - to the body. and started all good. Took it for a drive, all fine. Haven't had any problems since - will be getting a small Jump starter to keep in the wagon now. I did already have a larger one - with a compressor - but to be honest, I was scared to try jumping starting it - too many electrics for my liking. Would do it next time though.
I now know that the auxiliary battery gets charged when plugged into the main charger, but apparently won't charge it if it was flat when put on charge.