The importance of visually inspecting your equipment can not be underrated!
This battery was removed during a survey of a power boat. The owner claimed that the battery works fine, as he used it to start the generator yesterday! Turns out the generator is connected to this battery and in parallel with the engines starting batteries. When this battery died (and exploded), the owner never noticed because the generator kept starting.
The inside of the battery is severely corroded and all the cells are dry!
"Maintenance Free" doesn't mean "Inspection Free". Even if you don't need to add water to your cells, it is still a good idea to look at your equipment periodically to make sure you don't have something like this lurking beneath your cabin sole.