One thing Miata owners should do is clean the battery cables every so often. A loose or slightly corroded battery cable could produce a spark. although the AGM batteries use a gel with a glass mat absorber there is still gasses produced and if one of your rubber tubes is old and has even a small crack all it takes is a small spark to ignite the gas built up from being in an enclosed trunk. Just one small maintenance item Mazda did not put in their book was battery cable maintenance. I clean mine at least once a year. Also they sell battery grease to put on the terminals to prevent corrosion and it also helps keep any sparking at a minimum. You can also get those colored post felt pieces that are supposed to protect from corrosion, not to sure how effective they are but some people put them on their batteries.

This is not a cure but battery inspections could save you a lot of work in clean-up. If you have a wet cell battery check the water often especialy in the hot weather. Keep it filled to the bottom of the fill holes, do not over fill and do not let it get so low you can see the plates. This would cause flaking of the lead plates which settles into the bottom of the battery, shorten its life and possibly short across and drop your voltage which is not good for electronic equipment either.

Also just a reminder never add any more acid to the battery when filling, just add water as the acid is just fine, it is the water that evaporates. Also adding acid to water could cause a chemical reaction and fly back at you. "NEVER ADD ACID TO WATER, ALWAYS ADD WATER TO ACID"