Ive seen lots of Turbo cars spit out flames, not so many Naturally aspirated ones with engines in the front, Porsches tend to do occasionally due to the exhaust being so short. I would say if there was anything wrong and your engine was running the wrong air/fuel mixture and it wasnt burning up properly your 02 sensor would catch that and throw a code. It could be residual fumes that mixed with the perfect amount of air to ignite. If the car is running fine I wouldnt worry about it too much. Hope this puts you a little more at ease about the situation.
09 E92 M3- Sold
07 E92 335i- Sold