Mine isn't under the battery, rather above it in a small rack behind the battery control module, assuming they are all the same?!? If so not much chance of water collecting where is is and the cables run down from the module out of the car also..... A8 parts have a module in stock http://www.ebay.co.uk/itm/Audi-A8-D3.../170794371018?

If it's in there, it could have been disturbed in the jump start process. It's a proper factory fit, so I'd expect it to be in the 'proper' place.

Apologies, possible miss information on my part, you are quite right the controller is under the battery..... worth a look anyway and if it is foobared at least its easy to get to for replacement! :)