ACFT EVACUATED BY SEVERAL PAX AFTER APU START CREATES A FLASH OF FIRE ON EXTERIOR OF ACFT.

Narrative

AFTER WE PULLED INTO OUR GATE AT DTW ARPT; THE ACFT WAS CHOCKED AND EXTERNAL PWR WAS NOT AVAILABLE. I DIRECTED THE SO TO START THE APU; WHICH HE DID. AFTER THE APU WAS STARTED; SHIPS PWR WAS TRANSFERRED TO THE APU AND I SHUT DOWN #3 ENG AND DID A PARKING CHKLIST. AT THAT TIME THE LEAD FLT ATTENDANT CAME UP HURRYING TO THE COCKPIT TO INFORM ME THAT SEVERAL PAX SAW FLAMES ON THE R WING AND TOOK IT UPON THEMSELVES TO EVAC OUT OF THE L HAND OVERWING EXIT! APPROX 6 PAX LEFT THE ACFT UNAUTHORIZED; AND 3 DROPPED TO THE RAMP! I IMMEDIATELY WENT BACK TO THE MAIN CABIN AND DIRECTED THE PAX BACK TO THE ACFT. NONE WERE HURT; AND AFTER EVERYONE WAS ACCOUNTED FOR THE INCIDENT WAS OVER. THIS INCIDENT OCCURRED DUE TO WILD PANIC. HAD SOMEONE 'IN THE KNOW' ADVISED US EARLIER OF A 'HOT' APU I COULD HAVE MADE AN ANNOUNCEMENT TO ALLAY FEARS. I DID MAKE AN ANNOUNCEMENT BUT IT WAS TOO LATE; BY THE TIME THE PA WAS MADE; THE PEOPLE HAD LEFT THE ACFT. BUT; THEY APPARENTLY PANICKED AND IT SPREAD TO SEVERAL OTHER PEOPLE. I FEEL THE FLT ATTENDANTS SHOULD HAVE PUT A STOP TO THE UNAUTHORIZED EVAC EARLIER ON; AS WE IN THE COCKPIT HAD NO CTL OVER IT. PERHAPS THEY SHOULD INCLUDE SCENARIOS LIKE THIS IN FLT ATTENDANT TRAINING SINCE IT IS EASILY INITIATED BY JUST 1 PANICKED PAX. SUPPLEMENTAL INFO FROM ACN 188390: SOON AFTER PARKING CHKS COMPLETED SOMEONE (I BELIEVE A FLT ATTENDANT) RPTED THAT A FEW PAX HAD EXITED THE ACFT THROUGH THE L EMER WING WINDOW EXIT. APPARENTLY; A PAX HAD YELLED FIRE! DURING THE UNLOADING PROCESS. THAT PAX NEVER WAS IDENTED. THE CAPT PROCEEDED AFT; PASSING THE NORMALLY DEPLANING PAX; FINDING 2 PAX ON THE WING. HE ORDERED THEM BACK INSIDE. HE THEN WENT DOWN THE JET BRIDGE TO FIND THE 2 PAX WHO; ON THEIR OWN INITIATIVE; BEGAN AN EMER EVAC OUT THE WINDOW. NO INJURIES WERE RPTED. THINKING THERE MAY HAVE BEEN TORCHING UPON APU START UP; THE CAPT CHKED FOR FIRE INDICATIONS AROUND ACFT FROM THE GND. NO INDICATIONS WERE FOUND. ASKING SEVERAL RAMP PERSONNEL; NO ONE INDICATED SEEING ANY INDICATIONS OF A FIRE. AS WE; THE CREW; WAITED ON ACFT I PERSONALLY ASKED GND PERSONNEL IF THERE WERE FIRE INDICATIONS; NO ONE ANSWERED IN THE AFFIRMATIVE. NO ONE I TALKED TO; INCLUDING THE PAX WHO EXITED THROUGH THE WINDOW; CLAIMED SEEING FIRE INDICATIONS. THERE MAY HAVE BEEN TORCHING ON APU START UP (EITHER FIRE OR JUST SMOKE) BUT I CANNOT CONFIRM. THE EVENT WAS RPTED TO COMPANY SECURITY; AND FAA OFFICIALS (TWR PERSONNEL).