ACR FLC DEVIATED FROM RTE CLRNC WHILE CAPT WAS ABSENT FROM DUTY STATION.

Narrative

THIS WAS A 2 PLT ACFT WITH AUGMENTED CREW FOR OVER 12 HRS OF FLT. I WAS ONE OF THE EXTRA FO'S AND AT THE TIME OF OCCURRENCE I WAS TECHNICALLY OFF DUTY. THE CAPT VACATED HIS SEAT TO TAKE CARE OF PHYSIOLOGICAL NEEDS AND I CLBED INTO HIS SEAT SO THE OTHER FO WOULD NOT HAVE TO PUT ON THE OXYGEN MASK. AT THIS TIME WE SHORTLY CROSSED OVER THE OED VOR NBOUND AND ACFT HEADED DIRECTLY FOR FINGS INTXN AS THE AREA NAV FMC WAS PROGRAMMED. ABOUT 50 NM N OF OED; ZSE ADVISED US WE WERE L OF TRACK AND ASKED US TO VERIFY WE WERE ON J-501. WE INDICATED WE WERE DIRECT FINGS INTXN AND ATC ADVISED US THE DIRECT RTE WOULD TAKE US ACROSS RESTR MIL AIRSPACE AND ON OUR REQUEST GAVE US A 15 DEG R TURN BACK TO JOIN J-501. AS FAR AS WE KNOW THERE WAS NO TFC CONFLICT. UP TO THIS POINT I HAD NOT RECHKED THE MASTER FLT PLAN WHICH THE OTHER FO WAS IN CHARGE OF BECAUSE THE CAPT WAS FLYING THIS LEG. OBVIOUSLY I SHOULD HAVE AS SOON AS I CLBED INTO THE CAPT'S SEAT TO VERIFY OUR ROUTING. THE FLT PLAN DID SHOW J-501 TO FINGS INTXN NOT DIRECT. THE OTHER FO WHO HAD CHKED THE MASTER FLT PLAN AGAINST THE RTE SAID THAT HE HAD ONLY CHKED THE FMC RTE AFTER THE OCEANIC GATEWAY AND HAD NOT CHKED THE DOMESTIC PORTION AND 95 PERCENT OF OUR TRIPS GO DIRECTLY TO THE OCEANIC RTE WITHOUT A LONG DOMESTIC PORTION. HE ALSO HAD NOT STARTED THE USUAL WAYPOINT CHKING ENRTE YET BECAUSE WE WERE IN RADAR CONTACT ON A DOMESTIC RTE. HE AGREED THAT BOTH OF THOSE OMISSIONS WERE AN OVERSIGHT HE WOULD NOT REPEAT.