I had a similar problem with my Range Rover. If left un-driven for 2 days the battery would be flat. The cause of the problem was the signal from an external sensor for a weather station. When the car is locked the ECU checks that all circuits are inactive and then it enters sleep mode. Every time the weather station sensor emitted a signal the ECU would wake up and cause a drain on the battery. Moving the external sensor out of range of the car solved the problem.
