Turns out one of the CAP airplanes based at KAPA was doing the runup. Here's the report I got in email:
During pre-takeoff Run-up, aircraft (Sirus CR-22) encountered a nose well flash fire. Mission Observer Grabbed fire extinguisher, leaving aircraft to render assistance. All passengers safely egressed their aircraft and MO and one other were able to initially extinguish the fire before emergency responders arrived. See mission files for additional info.
Two scary observations went along with today's event.
One: it is frightening to see how fast a composite skinned aircraft can burn. Today's fire was small and lasted less then 60 seconds, yet the entire cowling was destroyed - composite lava! It took the fire department about five minutes or better to arrive on scene!
Second was fire extinguishers. The occupants of our burning aircraft either had no fire extinguisher or did not know how to use it. Next time you fly, take a few minutes and brush up on how to get to our extinguishers, both in the hangar and in the aircraft.
What we learned today: Fire can happen without any warning, fire extinguishers are life savers and the knowledge of how to use them is critical!