I thought the girlfriend might be causing the flat battery
Now on the serious side, did you have the Xenons on while you were washing the car? as mine do not work unless the engine is running, I have had the radio on for over 2 hours while working on my MINI and no flat battery.
I think you have Ipod thing and I read that they can cause excess drain on the battery as they do not allow the car bus system to go to sleep.
I would have the dealer look at the car (with out the Ipod thing installed) as there are other problems that can cause this and your dealer should fix them for you.
Snoopy