Your post regarding taking on water in your bmw 5 series wagon was very helpful and directed me to a new possibility regarding the cause of an identical problem in my 2006 530xi wagon. The dealership here states that I must have driven through high water, forcing water in through the rear baffles, hence the problem was cause by my driving. I am certain I did not go through such water. It's a small dealership and it appears they may not be too well informed.
It would be useful for me to know the name of the dealership where you took your car so that I can contact them to find out more about bmw's suggestions for servicing the sunroof drains. Any other advice you have on this will also be appreciated.