B737 FLC AND FLT ATTENDANTS SURPRISED TO FIND THAT A PAX WAS IN THE AFT LAVATORY DURING FINAL APCH PHASE OF FLT AND LNDG AT TPA; FL.

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LNDG WITH PAX IN LAVATORY. FLT LANDED AT TPA UNEVENTFULLY. AFTER SHUTDOWN; FLT ATTENDANTS INFORMED ME THAT THERE WAS A PAX STILL IN THE LAVATORY DURING OUR LNDG. THIS; DESPITE THE FACT THAT THE 'FASTEN SEATBELT' SIGNS WERE ILLUMINATED AND THAT I HAD MADE OUR ARR ANNOUNCEMENT PRIOR TO DSNDING THROUGH 10000 FT REMINDING THE PAX OF OUR IMMINENT APCH AND THAT THE SEATBELT SIGN WAS ILLUMINATED. I ASKED THE PAX TO 'INSURE THAT THEY REMAIN IN THEIR SEATS WITH THEIR SEATBELTS FASTENED FOR THE REMAINDER OF THE FLT.' I AM NOT BLAMELESS IN THIS MATTER. I FAILED TO MAKE THE 'SECURE THE CABIN FOR ARR' ANNOUNCEMENT WHICH WOULD HAVE ALERTED THE FLT ATTENDANTS TO BE MORE AWARE OF PAX GETTING UP AND GOING TO THE LAVATORY. AS IT WAS; THE FLT ATTENDANTS RECOGNIZED OUR IMMINENT LNDG AND QUICKLY TOOK THEIR JUMP SEAT IN TIME FOR THE LNDG. THE PAX; HOWEVER; REMAINED IN THE LAVATORY. I HAD TO CONFIGURE EARLY DURING THE APCH BECAUSE ATC VECTORED ME OFF MY DIRECT PATH TO THE IAF BUCCS TO FLOW A MUCH SLOWER ACFT. AS I RESPONDED 'COMPLETE' TO THE FLT ATTENDANT NOTIFICATION PORTION OF THE CHKLIST; I THOUGHT I HAD MADE THE ANNOUNCEMENT. IT WAS WHEN I HEARD THE FLT ATTENDANTS DROPPING THEIR JUMP SEATS WHILE ON SHORT FINAL THAT I REALIZED THAT I HAD FORGOTTEN TO MAKE THE ANNOUNCEMENT. MITIGATING FACTORS INCLUDE THE FOLLOWING: 1) WE WERE LATE AND THE HR WAS LATE -- WE HAD AN UNSCHEDULED ACFT CHANGE IN JACKSONVILLE. 2) THIS WAS OUR THIRD ACFT TYPE (500; 300; 700) IN 4 LEGS. 3) OUR APCH REQUIRED SOME UNUSUAL VECTORS AND EARLY CONFIGN IN ORDER TO MAINTAIN PROPER SPACING BEHIND A SLOW ACFT IN IMC WHICH CAUSED ENOUGH DISTR THAT I MISSED MAKING THE 'SECURE THE CABIN' AT MY NORMAL TIME; IE; 6000 FT OR 20 MI. 4) I 'THOUGHT' I HAD MADE THE ANNOUNCEMENT WHEN I RESPONDED TO THE CHKLIST. FUTURE PREVENTION IS SIMPLY A REAPPLICATION OF ATTN TO TASKS AND CHKLIST RESPONSES. DID I REALLY GIVE THE ANNOUNCEMENT OR WAS THAT LAST LEG? -- PARTICULARLY WHEN THE CREW IS GETTING TIRED AND IT'S LATE.