I cautioned above in an earlier post, the local racer in question is Tyler Hooks:
https://blog.teknorc.com/2018/08/24/tyler-hooks-wins-13-5-4wd-roar-nationals/
Anyway, ask him for more details if you like, but his garage was a total loss after a battery had spontaneously combusted within a few hours after he returned from the race track. No apparent damage from any of his batteries, no cracked cases, no swollen cells, all batteries were in good working condition and no notable crashes on the track. He stored all of his batteries together in a plastic rolling tool box. It created a chain reaction, fortunately the fire department was able to put out the fire in the garage before it had spread to the home, but all the contents of the garage and roof of the garage were completely burnt to the ground.
Takeaway for me are
1) Do not store any batteries close together so you don't risk a chain reaction
2) Keep batteries in a cool/dry place, preferably inside an HVAC home
3) Ensure the storage space can be ventilated (open window) in the rare event a fire erupts