Water Standing in Battery Compartment- Help

[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?

[Thik]

Maybe because you live under a waterfall?

...lol j/k

My battery compartment was flooded with water because of a water leak around the trunk area. I saw another M3 at my dealership having the same issue. Seems some M3s were not assembled correctly resulting in water leaking into the department. You'll need to take it to a mechanic where they will do a water test to see exactly where the water is coming in from, then they'll have to plug it somehow.

[Three Pedal Steve]

Originally Posted by Thik

Maybe because you live under a waterfall?

...lol j/k

This is why I love this forum!
I was so anxious to share my rant, it never occurred to me how this might appear. Next time I will close the sunroof before driving under my waterfall.

[dpaul]

Same problem on my 2011 E93 M3 - caused by incorrectly installed gasket around gas filler. Took some time to figure this out. I removed trunk trunk lining and ski bag pass then crawled into the trunk while my wife sprayed the car with a hose. In my case, the water corroded the main battery cable so the car wouldn’t start- otherwise I’m not sure I would have noticed

[SEMOM3]

I had the same problem as well. I found the leak coming from the taillight housing so I tightened the nuts a little more and the leak never came back. dpaul's troubleshooting method is the best way to figure it out.

[Zertag01]

Hello...I just replace the battery in the 2011 BMW M3 coupe on 11-17-2018 as I pull out the battery I notice water in the battery compartment, not much but I was wondering why and how this happened. This could be a problem in the future with corrosion.