A B757-200 CAPT; DURING A TRANSOCEANIC FLT; MISTAKENLY DID NOT FOLLOW THE PROPER SECURITY PROCS FOR LEAVING THE COCKPIT TO USE THE LAV.

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THE FREQUENCY BEING USED WAS HF. I COULDN'T GET THE COMPANY RPT PAGE TO TAKE ANY HF FREQUENCY NO MATTER HOW I ENTERED IT; SO I PUT 121.5 IN SINCE I WAS MONITORING IT DURING THE OVERWATER/HF PORTION OF THE FLT. OUR EXACT LOC AT THE TIME OF THE INCIDENT WAS ABOUT 100 NM S OF CHAMP. DURING CRUISE; I CALLED THE FLT ATTENDANTS ABOUT A LAV BREAK. THEY ENSURED THE FWD LAV WAS EMPTY; THE CART WAS ACROSS THE AISLE; AND THAT MORE THAN ONE FLT ATTENDANT WAS PRESENT (THERE WERE THREE). THEY THEN CALLED ME; INFORMED ME THE AREA WAS CLR; AND I CONFIRMED IT THROUGH THE VIEW PORT. I OPENED THE DOOR; HANDED THEM THE MEAL TRAYS; CLOSED THE COCKPIT DOOR BEHIND ME AS THE FLT ATTENDANT WAS ASKING IF I WANTED HER TO GO INTO THE COCKPIT? I SAID YES AS I REALIZED WHAT I HAD DONE. I QUICKLY USED THE LAV AND THEN INFORMED THE FO; VIA THE INTERPHONE; THAT I HAD MISTAKENLY CLOSED THE DOOR (HE HAD OBVIOUSLY ALREADY REALIZED THAT); AND I WAS GOING TO USE THE CODE TO OPEN THE COCKPIT DOOR. I RETAINED COM WITH HIM UNTIL THE DOOR TIMED OPEN; THEN I REENTERED THE COCKPIT AND RESECURED THE DOOR. THE FLT ATTENDANTS THEN MOVED THE CART AND LEFT THE GALLEY AREA. THERE IS A POSSIBLE SECURITY PROB WHEN ONLY ONE PLT IS IN THE COCKPIT. HAD THREE FLT ATTENDANTS NOT BEEN IN THE GALLEY AREA; THE CART NOT BEEN ACROSS THE AISLE; AND THE INTERPHONE NOT BEEN USED TO COORDINATE THE CODE OPENING OF THE DOOR; THERE WAS THE POTENTIAL OF A COCKPIT BREACH; HAD ONE BEEN PLANNED. THESE MEASURES LESSENED THAT POSSIBILITY DURING WHAT; I HOPE; IS THE ONLY TIME THIS EVER HAPPENS TO ME OR MY CREW.