CABIN ATTENDANT RPT; A300; MIA-SJU. SMOKE; THEN DISSIPATED; ON INBOUND PLANE. OUTBOUND FLT HAD SMOKE; ALL LAVATORY SMOKE ALARMS ON R SIDE SOUNDED. RETURN MIA. FLT CANCELED.

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THE CAPT HAD BRIEFED US BEFORE DEP THAT THE PREVIOUS CREW SMELLED SMOKE IN THE AFT GALLEY ON THEIR TKOF. AFTER WE TOOK OFF; THE PURSER CALLED ME TO ASK IF I DETECTED ANY SMOKE. I DID NOT. HOWEVER; WHILE TALKING WITH THE PURSER; THE SMOKE DETECTOR IN LAVATORY #2; IN FRONT OF MY JUMP SEAT; WENT OFF. ALSO AT THAT TIME; FLT ATTENDANT #4 RPTED SMELLING A STRONG SMOKE SMELL AT HIS JUMP SEAT. ALL SMOKE DETECTORS ON THE R SIDE OF ACFT WENT OFF. I ASKED FOR A HALON FIRE EXTINGUISHER AND CHKED LAVATORIES FOR FIRE. THERE WAS NO FIRE AND NO SMOKE SMELL EITHER. ALL WAS RPTED TO THE COCKPIT. WE TURNED AROUND AND LANDED IN MIA AT XB00. FIRE TRUCKS MET THE FLT ON THE TXWY AND THEN WE WENT TO THE GATE. THERE WERE NO INJURIES TO PAX OR CREW. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT THE CAPT WHO HAD BROUGHT THE PLANE IN FROM SAN JUAN TOLD THEIR CAPT THAT SHORTLY AFTER TKOF THEY EXPERIENCED A STRONG PLASTIC BURNING SMELL; BUT THE SMELL DISSIPATED AFTER 10 MINS. WHEN THIS CREW LEFT MIAMI; THE CAPT CALLED BACK TO HER JUMP SEAT AT DOOR 3R TO ASK IF SHE SMELLED ANYTHING; AND AS SHE WAS ABOUT TO SAY 'NO'; THE SMOKE ALARM WENT OFF IN THE AFT LAVATORY; THEN SUDDENLY; ALL THE ALARMS WENT OFF. WHEN THEY RETURNED TO MIAMI; THEY TOOK THE PLANE OTS FOR A MAINT CHK. 1 WK LATER; SHE FLEW WITH THE SAME CAPT AND THE SAME PLANE. NOTHING MECHANICALLY WRONG HAD BEEN FOUND WHEN THE PLANE WAS OTS. THEY DID NOT HAVE ANY PROB WITH SMOKE AGAIN EITHER; SO THE SOURCE OF THE SMOKE REMAINS A MYSTERY.