Moisture destroyed control module behind rear seat?

Our 2018 Q5 with 25K miles suddenly died. FJ Audi in Chicago determined control unit was exposed to moisture and corroded. Evidently this is located behind rear seats. We have no recollection of any spilled drinks? Is this a design defect? Any other way this can happen? Expensive repair not covered by warranty! I found other reports on Audi forums, but I figured I should try here too.