2010 is 350 backfire
 

My mother-in-law drives a 2010 IS 350, 23K miles, stock everything, always maintained by the dealer and runs only Chevron Premium gas. It has now twice backfired (very loudly) on her. Once this week and once a few months ago. Both times were as she was driving around 35mph, neither accelerating or decelerating. Dealer can find no fault codes and took a "snap shot" of it running and saw that all sensors, etc. were operating correctly.

Has anybody heard of such a thing? Or any clue what might be wrong?

Thanks