GLF2 CREW HAD A TRACK DEV WHILE FLYING THE KEPEC RNAV ONE ARR AT LAS.

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ON KEPEC ONE ARR TO LAS; I WAS HAND-FLYING THE ACFT DUE TO CONTINUOUS LIGHT TO OCCASIONALLY MODERATE TURB; THIS IN AN ATTEMPT TO PROVIDE A SOMEWHAT SMOOTHER RIDE FOR THE PAX SINCE THE AUTOPLT DOES NOT HAVE A 'SMOOTH RIDE' FUNCTION. I WAS ALSO VISUALLY SCANNING FOR OTHER TFC IN THE AREA. FO WAS COMPLETING THE DSCNT/IN-RANGE CHKLISTS. I SAW OTHER TFC CLBING OUT OF LAS IN THE DISTANCE; AND DETERMINED THAT THE TFC WOULD NOT BE A FACTOR TO US. AFTER A FEW MOMENTS; I SENSED THAT WE SHOULD HAVE TURNED BY NOW. LOOKING DOWN TO THE FMS; I REALIZED THAT; FOR SOME REASON; THE FLT DIRECTOR HAD NOT INDICATED THE TURN; BUT RATHER ACTED AS IF SELECTED TO HEADING MODE. I IMMEDIATELY STARTED THE L TURN. A FEW MOMENTS LATER; ATC TOLD ME TO TURN L TO HEADING 340 DEGS. I QUERIED MY FO WHETHER HE HAD PUSHED ANY FLT DIRECTOR SELECTOR KNOBS OR DONE ANYTHING TO THE FMS. HE WAS BAFFLED AND DID NOT UNDERSTAND WHAT COULD HAVE HAPPENED. AFTER LNDG; I CONTACTED LAS APCH; SPOKE WITH A VERY COOPERATIVE CTLR; AND EXPLAINED WHAT HAD HAPPENED. DURING MY APCH; I RECEIVED AN ERROR MESSAGE ON THE FMS AND I PROCEEDED TO WRITE IT UP AS A SQUAWK. I PRESENTLY HAVE OUR COMPANY'S MAINT DEPT INVESTIGATING THE AFFECTED FMS AND CONDUCTING ANY NECESSARY REPAIRS. I SEE THIS EVENT AS AN EXCELLENT REMINDER NOT TO TRUST THE MODERN FMS SYS 100%; BUT RATHER TO MAINTAIN A CONTINUOUS AND VERY CAREFUL EYE ON THE EQUIP ESPECIALLY IN TERMINAL AREAS.