Pretty sure the battery is vented to the outside.Overcharging the 12v battery will cause the smell and even though it tested fine at the time it may have been damaged (swelling maybe) so over some time failed because of that.
If this ever happens again lift the back seat and the cover over the battery and check for a stronger smell in that area. As for testing the battery it may be a good idea to remove it and look for swelling.
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