Glad all is ok.Would it possibly be a case of over charging? Faulty charger?I have personally seen this happen to a over charged battery.
Make sure you wash down and rinse out the compartment well and rinse the bilge well.
Hope you got a fire extinguisher on borad too cause you said you got 3 more??
I have a 2 bank onbaord charger from cabelas made by Pro Mariner. I bought them last year. How can you tell if its over charging?