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.