I have now been able to confirm this incident occurred.
The ATSB occurrence number is 200901475. It was a Cat 5 occurrence so it does not have its own page. It is listed in
this document at the top of page 15.
Apparently it is a proven fact that water bottles can cause fires - refer
this article.
Amazing!
I've also seen the damage caused by a pair of glasses left on the top of a vehicle dash board when the wearer changed to sunnies.