A320 FLIGHT CREW REPORTS UNCOMMANDED CARGO COMPARTMENT FIRE BOTTLE DISCHARGE DURING BAGGAGE UNLOADING.

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WE BLOCKED IN AND FINISHED THE CHKLIST. WE TURNED OFF ALL OF THE SCREEN PER GO DARK PROC. I HAD ALREADY GATHERED MY THINGS WHICH WERE STACKED BEHIND MY FO SEAT AND I WAS UP STANDING IN THE DOORWAY SAYING GOODNIGHT TO THE PAX. I WAS ABOUT TO GO INTO THE LAVATORY WHEN I HEARD THE FIRE BELL GO OFF IN THE COCKPIT. I LOOKED BACK IN AND SAW THE MASTER WARNING LIGHTS FLASHING. THE CAPT WAS STILL IN HIS SEAT; SO HE TURNED THE SCREENS BACK ON SO WE COULD SEE WHAT WAS GOING ON. IT TOOK 10 SECONDS OR SO FOR THE SCREENS TO COME BACK ON. WE SAW WE HAD AN AFT CARGO FIRE INDICATION. CAPT DIRECTED ME TO GO CHK IT OUT. SO I HEADED FOR THE JETWAY STAIRS. AS I WAS AT THE DOOR TRYING TO FIGURE OUT WHICH CODE TO USE; THE RAMPER CAME UP THE STAIRS. HE OPENED THE DOOR AND SAID THE FIRE EXTINGUISHER HAD JUST GONE OFF IN THE AFT PIT. I ASKED IF THERE WAS ANY FIRE AND WE BOTH HEADED DOWN THE STAIRS. THE RAMPER SAID THERE WAS NO FIRE. THE CAPT OPENED HIS SLIDING WINDOW AS I GOT TO THE NOSE AND ASKED IF THERE WAS A FIRE. I SAID NO; AND WENT BACK TO THE PIT. THE DOOR WAS FULLY OPEN AND THE BELT LOADER WAS UP TO THE DOOR. I DID NOT SEE ANY FIRE AND I WENT UP THE LOADER AND CRAWLED INTO THE PIT. I LOOKED AROUND FOR ABOUT 20 SECONDS TO MAKE SURE THERE WAS NO SMOKE OR FIRE. I WENT BACK TO THE COCKPIT AND TOLD THE CAPT WHAT I HAD FOUND. WE LOOKED AROUND AND SAW THE DISCHARGE #1 LIGHT ILLUMINATED ON THE CARGO FIRE PANEL. BOTH BOTTLE SWITCHES WERE STILL TOTALLY GUARDED AND IN THE NORMAL POSITION. THE CAPT SAID HE LOOKED AT THE FLT MANUAL FOR THE AFT CARGO FIRE AND IT SAID TO NOT FIRE ANY BOTTLES ON THE GND. SOMEHOW THE BOTTLE HAD DISCHARGED. WE WROTE UP THIS INCIDENT AND CALLED MAINT CTL. WE CHKED WITH THE RAMPERS AND THEY SAID EVERYONE WAS OK AND THEY WOULD GO AHEAD AND UNLOAD THE PIT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE REPORTER WAS NEVER INFORMED OF WHAT MAY HAVE CAUSED THIS FIRE BOTTLE TO DISCHARGE ON ITS OWN. AT THE TIME OF THE DISCHARGE THE GROUND CREW WAS JUST OPENING THE DOOR AND NO PERSONNEL WERE IN THE CARGO COMPARTMENT AT THE TIME. IT SEEMS THAT THE GROUND CREW WAS NOT INVOLVED IN THE BOTTLE DISCHARGE AND THE REPORTER REITERATED THE FLIGHT CREW'S LACK OF INVOLVEMENT.