B737-500 CAPT RECEIVED CALL TO RETURN TO ARPT. GND CREW BELIEVED ACFT ENG WAS STILL RUNNING. ENG WAS ACTUALLY MOTORING FROM APU BLEED AIR. BLEED VALVE CLOSED AND MOTORING CEASED.

Narrative

ARRIVED AT GATE IN BUF AT XA48 WITH L ENG RUNNING. RAMP PLUGGED IN GND PWR; BUT ACFT WOULD NOT ACCEPT. STARTED APU AND CONNECTED GENERATOR TO BUSSES AND PERSONALLY SHUT DOWN L ENG. FO TURNED ON APU BLEED AND L PACK. WE COMPLETED PARKING CHKLIST AND SECURED ACFT FOR OVERNIGHT WITH APU RUNNING. WE THEN WENT TO HOTEL; WHICH TOOK APPROX 20 MINS. UPON ARR AT HOTEL; I WAS GIVEN THE PHONE WHERE A FRANTIC RAMP WORKER FROM BUF WAS TELLING ME WE HAD LEFT AN ENG RUNNING. WITH COMPLETE CERTAINTY THAT WE HAD SHUT THE ENG DOWN; I STILL FELT I HAD NO CHOICE BUT TO PROCEED BACK TO ARPT AND ADDRESS CONCERNS. I DID; ARRIVING AT ACFT AT APPROX XB30. THERE; I FOUND THE L ENG ROTATING BUT NOT RUNNING. I WENT TO COCKPIT AND CONFIRMED START LEVERS WERE IN CUTOFF AND FUEL FLOWS WERE 0. I THEN TURNED OFF APU BLEED AND PACK. THEN THE RAMP AGENT THAT HAD CALLED ME AND I WENT OUTSIDE WHERE THE ENG WAS STILL TURNING BUT APPEARED TO BE SLOWING DOWN. HE SHOWED ME HOW THERE WAS AIR AND HEAT COMING FROM EXHAUST OF ENG PROMPTING HIM TO BELIEVE IT WAS RUNNING. THE ENG FINALLY CAME TO A STOP APPARENTLY AS A RESULT OF TURNING OFF APU BLEED. I CALLED MAINT AND CONFERRED WITH 2 TECHNICIANS WHOM ALONG WITH ME; HAD NEVER HEARD OF A CFM56 CONTINUING TO MOTOR WITHOUT BEING COMMANDED TO. THERE WAS NO INDICATION IN COCKPIT THAT START VALVE WAS OPENED BUT THIS APPEARS TO ME WHAT WAS HAPPENING. ONCE APU BLEED WAS CLOSED; ROTATION CEASED.