My mate had a similar thing happen in his old car.
The original classic Mini
Mini
Had the battery in the boot, not in the engine bay. I was supposed to be covered over, but my mate had taken it out to charge the battery and never replaced it.

He also had a can of de-icing spray in the boot. Can you see where this is going?

One feisty bit of cornering later and all of a sudden there was a hiss and a weird chemical smell. SHIT!

After a very quick emergency stop we were -fortunately- stupid enough to investigate the boot and then wildly kick at it with our young flailing gangly legs.

The battery cover was put over the battery from then onwards.