AS ONE B757 CREW MEMBER RELIEVED THE OTHER FOR LAVATORY BREAK; THE FLT CTLS WERE MOMENTARILY LEFT UNATTENDED.

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DURING CRUISE; FO WENT TO CABIN TO USE LAVATORY. FLT ATTENDANT SECURED COCKPIT DOOR. FO CALLED ON INTERPHONE TO RETURN. AT THAT TIME; I (THE CAPT) GOT UP; UNSECURED THE DOOR; AND LET THE FO IN. I PASSED HIM ON MY WAY TO THE LAVATORY. WHEN I RETURNED AND AFTER THE FLT ATTENDANT HAD LEFT; THE FO ASKED ME IF I KNEW WHAT I HAD DONE. I DIDN'T; SO HE INFORMED ME THAT FOR A FEW SECONDS NO PLT WAS IN EITHER OF THE PLT SEATS. MY RESPONSE WAS INCREDULOUS; UNTIL I REALIZED HE WAS CORRECT. I STILL CAN'T BELIEVE I DID THAT; BUT IT'S TRUE. I AM VERY CONSCIENTIOUS ABOUT SECURITY AND SAFETY AND DON'T KNOW HOW I COULD HAVE DONE THIS -- BUT I WONT' FORGET THAT I DID IT. SUPPLEMENTAL INFO FROM ACN 545161: I CALLED ON THE INTERPHONE TO RETURN. WHEN THE COCKPIT DOOR OPENED; I WAS VERY SURPRISED TO SEE THE CAPT HAD OPENED THE DOOR. RATHER THAN CREATE A SCENE AT THAT MOMENT; I QUIETLY OCCUPIED MY SEAT AS QUICKLY AS POSSIBLE. WE DISCUSSED TOGETHER THE PRIMARY REASON FOR THE FLT ATTENDANT BEING IN THE COCKPIT TO OPERATE THE DOOR AND LOCK SO THAT A PLT'S SEAT IS NEVER UNOCCUPIED.