In case it might help anyone, my Range Rover Sport (2006) would always have a flat battery if I left it unused for more than about one day.

To cut a long story short, it was finally diagnosed to the upper tailgate window release catch. It was completely corroded. It had turned to white powder inside and had stuck in the live position.

We replaced it with a new one the problem was gone. I think this is a common problem with these release catches. Its a Ford part and its used on some Discoverys as well I think.

So if you have a battery drain problem and you cannot open the upper tailgate, this might help.