My mate had a similar thing happen in his old car.
The original classic 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.
My mate had a similar thing happen in his old car.

The original classic 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.