I’m not sure if this belongs in the Repair and Maintenance section, but here goes.
A few years ago I was attending an auction that included some antique cars. While I was in that section, I kept smelling a rotten egg smell, and I was trying to figure out where it was coming from, so I went over to a car that had the hood up. I heard somewhat of a quiet rumbling/boiling sound, but I couldn’t figure out what was causing the smell.
A little bit later, someone noticed a little bit of smoke coming from the engine area and hurried up and unplugged the old fashioned battery charger. Apparently what I heard boiling was the battery!
I have occasionally wondered after that day how close I came to having something catastrophic happen when I was near the car.