MD11 FLC DECLARED AN EMER AND RETURN LAND AFTER HAVING THE LAVATORY SMOKE DETECTOR ANNUNCIATOR LIGHT COME ON FOR THE CREW LAVATORY.

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CLBING THROUGH 10000 FT TO 10500 FT MSL; A SMOKE DETECTOR ACTIVATED JUST AFT OF THE COCKPIT. THE 'LAVATORY SMOKE' ALERT DISPLAYED ON THE EAD. WHILE THE FO AND I BEGAN THE ABNORMAL PROC IN THE POM; I ASKED JUMP SEAT RIDER IF HE WOULD INVESTIGATE. IN APPROX 1 MIN; ONE OF THE FLT ATTENDANTS ENTERED THE COCKPIT TO SAY THAT THE ACTIVATED DETECTOR WAS IN THE CREW REST FACILITY (JUST AFT OF THE COCKPIT DOOR). SHE THEN OBSERVED THAT SHE COULD SMELL AN ODD ODOR. BOTH THE FO AND I AGREED THAT WE COULD SMELL IT ALSO. IT SMELLED 'HOT.' WITH THE SMOKE DETECTOR STILL ACTIVATED; I DECIDED TO RETURN IMMEDIATELY TO JFK TO INVESTIGATE. AS A PRECAUTION; I DECLARED AN EMER SO THAT WE COULD LAND ASAP AND ALSO TO ENSURE THAT THE FIRE TRUCKS WOULD GREET US ON LNDG. DURING DSCNT; THE DETECTOR SILENCED BUT REACTIVATED JUST PRIOR TO LNDG. ALSO; JUST PRIOR TO LNDG; I OBSERVED THAT THE #1 PACK FAILED. THE LNDG AND SUBSEQUENT TAXI WERE UNEVENTFUL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT HE HEARD FROM COMPANY MAINT THAT ONE OF THE BLEED AIR PACKS HAD MALFUNCTIONED CAUSING A LACK OF CABIN PRESSURIZATION WHICH IN TURN CAUSED THE ALT CABIN ALERT LIGHT TO ACTIVATE. HE FURTHER STATED THAT THE CREW LAVATORY WAS FOLDED; OR COLLAPSED LIKE AN ACCORDION; SO THAT NO ONE COULD HAVE BEEN USING IT. ALSO; THE #1 BLEED AIR PACK IS UNDERNEATH THE AFT COCKPIT WHERE THE CREW LAVATORY IS LOCATED FROM WHICH THE HEATED FUMES WERE COMING. THEREFORE; SINCE THE SMOKE DETECTOR IN THE CREW LAV IS CONNECTED TO THE SMOKE DETECTOR SYS; THE FUMES WERE COMING FROM THE BLEED AIR PACK. HE HAS ALSO EXPERIENCED THIS IN OTHER ACFT.