A B757-200 IN CLB HAD DOOR 4L UNLOCK WARNING WITH FLT ATTENDANT'S RPT OF DOOR HANDLE MOVING UPWARDS AND TOWARDS THE OPEN POS.

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OUTSOURCED 'C' CHK. FIRST FLT POST 'C' CHK. DOOR 4L (AFT-MOST L) ENTRY/SVC DOOR INDICATED OPEN ON CLBOUT. FLT ATTENDANT AFFIRMED DOOR HANDLE ROTATED L FROM NORMAL-ARMED 3 O'CLOCK POS TO APPROX 1 O'CLOCK POS AND CREEPING LEFTWARD. FLT DECK INSTRUCTED FLT ATTENDANTS TO STOW/SECURE EVERYTHING; NOT TO TOUCH 4L DOOR AND MOVE AS FAR AWAY AS POSSIBLE. SECURE INTO JUMP SEATS. FO MADE ANNOUNCEMENT TO PAX THAT WE WERE TURNING BACK BECAUSE OF 'PRESSURIZATION PROB FROM A DOOR SEAL;' NOT THAT WE HAD 4L DOOR OPEN INDICATED. NOW ON SHORT FINAL; I MADE THE DECISION TO RETURN TO THE 'UNMANNED' DOOR 4L FOR THE FOLLOWING: 1) TURBULENT BUMP ON APCH OR HARD LNDG COULD COMPLETELY OPEN 4L; ACTIVATING PNEUMATIC ASSIST/SLIDE DEPLOYMENT. 2) PAX MAY THEN INITIATE UNWARRANTED EVAC FROM MOVING ACFT/JETBLAST = INJURIES/DEATHS. 3) FO ALREADY INDICATING '4L OPEN' WOULD BE UNAWARE OF THIS. 4) #3 FLT ATTENDANT (AT 3R DOOR) WOULD BE HELPLESS WITH 54 PAX BTWN HER AND REAR EXITS. UPON LNDG; 4L REMAINED STATIC. AS WE TURNED ONTO TXWY; DOOR HANDLE COMPLETED ITS ROTATION INDEPENDENTLY AND DOOR OPENED 5 INCHES. DAYLIGHT STREAMED IN. I GRABBED THE HANDLE; PULLED THE DOOR BACK IN AND HELD IT IN PLACE UNTIL RELIEVED BY MECH AT THE GATE. PNEUMATIC ASSIST/SLIDE DEPLOYMENT AVERTED BARELY. FAULT FOUND: IMPROPERLY HUNG/RIGGED DOOR ALIGNMENT BY OUTSOURCED MECH ON OUTSOURCED 'C' CHK. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THIS INCIDENT ACTUALLY TOOK PLACE ON THE SECOND FLT OUT OF A 'C' CHK. THE RPTR SAID THE FIRST FLT HAD A WRITE-UP ON THE DOOR WARNING ON DOOR 4L AND WAS SIGNED OFF AFTER WORK WAS ACCOMPLISHED. THE RPTR STATED THAT DURING CLBOUT THE FLT ATTENDANT AT DOOR 4L OBSERVED THE DOOR HANDLE MOVING FROM THE CLOSED POS UPWARDS TOWARD THE UNLOCK POS. THE RPTR SAID THE FLC RETURNED TO THE FIELD AND ON LNDG THE DOOR WENT PEN 5 INCHES. THE RPTR STATED THAT MAINT HAS RELEASED NO INFO ON THE MAINT ACTION TAKEN OR THE COMPONENT FAILURE.