A B737-300 IN CRUISE AT FL350 HAS FIRE BELL SOUND AND ENTIRE CARGO BAY FIRE WARNING SYSTEM LIGHT UP AS EXPECTED WHEN BEING TESTED. AFTER 10 SECONDS THE TEST STOPPED AND LIGHTS EXTINGUISHED.

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AT CRUISE FL350; 50 NM WEST OF ZZZ; THE FIRE BELL SOUNDED. I SILENCED THE BELL AND SAW THE ENTIRE CARGO BAY FIRE WARNING SYSTEM LIT UP; JUST AS IT WILL DO WHEN TESTED; INCLUDING THE ALPHANUMERIC DISPLAY LIT SOLID RED AS OPPOSED TO STATING THE FIRE LOCATION IN TEXT. I CALLED FOR THE CARGO BAY FIRE WARNING CHECKLIST. AS THE FO WAS OPENING THE CHECKLIST; THE ENTIRE TEST EVENT STOPPED. THE SYSTEM OK LIGHT WAS ON; AND ALL OTHER LIGHTS WERE OUT. THE SYSTEM WAS THEN TESTED AND IT TESTED NORMAL. THE FLIGHT ATTENDANTS WERE NOTIFIED. WE LET THEM KNOW OF THE TEST EVENT; AND ASKED THEM TO QUIETLY CHECK THE CABIN FOR ANY POSSIBLE SMOKE OR SMELLS. THEY REPORTED BACK THAT THERE WAS NOTHING UNUSUAL. WE CALLED DISPATCH/MAINTENANCE. WE AGREED THAT WE SHOULD CONTINUE TO ZZZ.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THIS WAS THE FIRST CARGO COMPARTMENT FALSE WARNING EXPERIENCED ON THIS AIRPLANE. THE EVENT LASTED FOR ONLY 10 SECONDS AND THEN RETURNED TO NORMAL. WHEN TESTED IN FLT AFTER THE WARNING; IT TESTED NORMALLY. ON LNDG; MAINT CHKED THE FORWARD AND AFT CARGO COMPARTMENTS AND FOUND NO EVIDENCE OF FIRE OR OVERHEATING AND THE SYSTEM AGAIN TESTED OK. WHEN ADVISING FLT OPERATIONS OF THE EVENT; THE RPTR WAS ADVISED THE FALSE WARNING HAD OCCURRED PREVIOUSLY AND WAS BELIEVED TO HAVE BEEN CAUSED BY A FAILED CARGO COMPARTMENT FIRE WARNING MODULE.