[Three Pedal Steve]

Has any other M3 owner experienced a flooded battery compartment/box (behind RH panel in trunk)? It was completely full of water, probably 10 inches deep. Further inspection revels standing water in the compartment behind the L H access panel as well. What appears to be an amp was completely submerged in water. Photo on left is battery compartment after removing battery and sucking out the water. Photo on right is the LH compartment after being drained. Discovered when airbag warning came on. Car runs fine.

I purchased the car new in 2013, it is always kept in a dry garage, and has never been in a flood. It is a moonroof car- both rear roof-drains appear unblocked. It still has the original battery. It is my totally pampered baby. Imagine my horror at this discovery. Questions:
1. Happened to anyone else?
2. Does an AGM battery emit corrosive chemicals?
3. Ideas on cause of such a thing?