My Audi A6 with the 2.7L Twin Turbo did something weird tonight. It had been sitting for maybe 1-1/2 hours and when I went to start it I heard something like a PS pump struggling then a pssst pssst and I saw light smoke from the left side??? People near by said that flames shot out from under the car with both psst's

Uh WTF? Any ideas? Everything was normal after that. What the heck would be combustable under there? let alone what ignited it. Maybe they thought I was purging my NOS

That's what it sounded like, only with flames. Weird as hell.
Just noticed I didnt say that the flames were from under the motor, not in the rear.