A DC10-30 ON A FERRY PERMIT HAS CARGO FIRE WARNING AFTER TKOF. AFTER RETURN TO DEP ARPT AND A MAINT CHK OF THE SYS; A REDISPATCH IS DONE WITHOUT AN UPDATED FERRY PERMIT AND A POSSIBLE INCORRECT DEFERRAL OF THE FIRE WARNING SYS.

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ON DEP FROM LAX; FERRY FLT XX MASTER WARNING LIGHT ILLUMINATED ON THE PLT'S GLARE SHIELD AND AS IT WAS CALLED OUT BY THE CAPT; I LOOKED UP AND SAW THE AFT CARGO COMPARTMENT RED WARNING LIGHT. IT WAS VERIFIED BY THE CAPT AND HE CALLED FOR THE CARGO FIRE CHKLIST AND I ACCOMPLISHED THE CHKLIST. WE RETURNED TO LAX; ADVISED MAINT WE WERE RETURNING. THE FIRE LIGHT WENT OUT WITH COMPLETION OF THE CHKLIST. THE REST OF THE ARR WAS NORMAL WITH CHKLIST READ. OUR MISTAKE WAS TAKING OFF FROM LAX WITHOUT A NEW RELEASE AS THE FERRY PERMIT DOESN'T ALLOW REDISPATCH ON THE SECOND DEP. ANOTHER MISTAKE IS WE DID NOT KNOW WE WERE ON A FERRY PERMIT IN THE FIRST PLACE. A BALL WAS DROPPED BY DISPATCH. WE WILL BE MORE VIGILANT IN NOTING OUR FERRY PERMIT IF THIS ARISES AGAIN. SUPPLEMENTAL INFO FROM ACN 488017: ACFT YY RETURNED TO MAINT AT LAX AFTER THE AFT CARGO FIRE LIGHT CAME ON INFLT. AFTER ACFT RETURNED; I ACCESSED THE AFT AND MID CARGO COMPARTMENTS AND FOUND NO EVIDENCE OF A FIRE OR GAS FROM A BOTTLE DISCHARGE. BOTH THE AFT AND MID CARGO COMPARTMENTS WERE CLEAN AND EMPTY OF ANY CARGO. I CALLED MAINT CTL AND READ THE LOGBOOK ENTRY AND LOG PAGE TO MAINT CTL. AFTER SOME DISCUSSION; DECIDED TO PLACARD PER MEL 26-17-01 (FIRE LIGHT INOP). FE; AFTER REVIEWING THE INCIDENT; IT MAY HAVE BEEN APPROPRIATE TO USE MEL 26-19-2 AND I SHOULD HAVE CHKED TO SEE IF IN FACT THE FIRE BOTTLE HAD BEEN FIRED AND TAKEN STEPS TO CORRECT IT.