HEADING DEV BY B737-300 NEAR ROD.

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WHILE IN CRUISE; FLYING FROM THE W TO E COAST; THE FO LEFT THE FLT DECK TO USE THE RESTROOM. I NOW BECAME THE PF DURING HIS ABSENCE. SHORTLY THEREAFTER ATC CLRED US DIRECT TO ROD VOR. I SELECTED ROD ON THE FMC TO GO DIRECT. SHORTLY THEREAFTER THE FO RETURNED TO THE FLT DECK. ATC THEN AGAIN ASKED US TO GO DIRECT TO ROD; WHICH WAS STILL 10 DEGS TO OUR L. AT THIS TIME I REALIZED THAT THE MODE CTL PANEL WAS STILL IN THE HEADING MODE FROM A PRIOR CLRNC. I THEN ENGAGED THE LNAV FUNCTION ON THE MODE CTL PANEL AND WE CONTINUED ON DIRECT TO ROD. THIS WAS A SIT WHERE I HAD ALLOWED MYSELF TO GET OUT OF THE LOOP AS A PNF. WHEN I SUDDENLY HAD TO TRANSITION TO THE PF; I FORGOT WHICH MODE THE FMC WAS IN. ADDITIONALLY; NOW I DIDN'T HAVE THE BENEFIT OF MY FO AS A BACKUP. FORTUNATELY; OUR DEV WAS MINOR AND PRESENTED NO CONFLICT WITH OTHER ACFT; BUT THE POTENTIAL WAS CERTAINLY THERE. THE MORAL TO MY STORY IS 'CHK AND RECHK' WITH EVERY CLRNC; ESPECIALLY WHEN BY YOURSELF. ADDITIONALLY; STAY IN THE LOOP WITH THE PF/PNF AT ALL TIMES. WHILE IT'S EASY TO BECOME COMPLACENT WHEN FLING LONG-HAUL FLTS; WE HAVE TO KEEP OURSELVES AWARE OF EVERY AND ANY CHANGE ON THE FLT DECK; IN ALL PHASES OF FLT.