This is not a hoax and it can happen.
Remember some years ago in school, when our Physics teacher took us out into the sun to practice what looked like sorcery at the time .It was not sorcery, it was just Physics.
What did she do ?. She burnt a piece of paper with a lens !!. This same thing can happen to your car.
How does this happen?
When light passes through a bottle of water (depending on the design and shape), it can converge and concentrate the light rays on one spot, in Physics this process is called “Refraction”.
The sun is very bright and hot. If it happens that the rays are concentrated on a spot that energy on your car seat, would ignite within seconds by, if all the conditions are right.
What should you do to prevent this?
Always keep bottles of water (especially the transparent ones) away from sunlight, its better to cover it completely or put it in a bag because the direction of the sunlight might change before you get back to your car.
Avoid keeping your bottle water your dashboard or seat.
Try as much as possible to pack your car under a shade
Don’t be like that man who opened his car and noticed that smoke was coming out of his chair, on proper inspection he noticed his bottle water was the culprit. If he had not gotten there on time, his car would have been engulfed in flames and as usual, village people would be blamed.