A CE550 MASTER WARNING LIGHT ILLUMINATED ON DEP WITH NO OBVIOUS CAUSE. WHILE TROUBLESHOOTING; THE CREW MISSED A REQUIRED DEP HDG CHANGE.

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ON THURSDAY WE WERE GIVEN A DUTY ON TIME OF XA00Z AT THE FBO. I WAS LISTED AS THE PIC FOR THE FLT. MY PARTNER WAS LISTED AS SIC. MY PARTNER WHO IS TYPED IN THE C500 WAS SITTING IN THE L SEAT AND TO BE THE SOLE MANIPULATOR OF THE CTLS FOR THIS FLT. I WAS TO PERFORM ALL THE PNF DUTIES. WE HAD AN XB41 DEP FOR A FLT FROM ZZZ1 TO ZZZ2. WE WERE EMPTY IN A C550 TO PICK UP A PAX IN ZZZ2 TO THEN FLY THEM TO ZZZ3. A FLT WE WERE SUPPOSED TO PERFORM ON THE PREVIOUS DAY HAD US FILL THE TANKS OF THE PLANE TO FULL. A NORMAL PREFLT WAS PERFORMED AND NO DISCREPANCIES WERE NOTED. WE HAD THE FBO CONNECT A GPU AND WE PROCEEDED TO DO ALL THE NECESSARY PREFLT CHKS; OBTAIN PERFORMANCE NUMBERS; SUBMIT A WT AND BAL AND COPY AND READ BACK OUR FLT CLRNC. WE THEN ENTERED THE FLT PLAN INTO THE PLANE'S GPS. AT APPROX XB40Z; WE TAXIED FROM FBO IN ZZZ1 TO RWY 8R. WE REVIEWED THE DEP AGAIN WHILE HOLDING SHORT OF THE RWY FOR TKOF. WE WERE GIVEN THE CLRNC TO POS AND HOLD ON RWY 8R; THE TWR ASKED US FOR OUR FIRST FIX AND I REPLIED WITH ABCDE. ZZZ1 TWR THEN TOLD US THAT AT THE MIDDLE MARKER TURN L TO 070 DEG HDG. WE REPEATED THE INSTRUCTIONS AND PERFORMED A NORMAL TKOF. ONCE THE GEAR AND FLAPS WERE UP; WE WERE WAITING ON THE MIDDLE MARKER ANNUNCIATOR LIGHT TO FLASH. I NEVER SAW THE MARKER GO OFF AND WHEN TWR SWITCHED US FROM THEM TO DEP; THE MASTER WARNING ANNUNCIATOR STARTED FLASHING. I REMEMBER THINKING TO TELL MY PARTNER TO MAKE THE TURN; BUT CANNOT REMEMBER IF I TOLD OR MOTIONED HIM TO DO SO. I IMMEDIATELY STARTED LOOKING FOR THE SOURCE OF THE WARNING LIGHT AND FOUND NO OTHER INDICATIONS ON THE PANEL. ALL THE ENG INSTS WERE FINE AND THE ELECTRICAL GAUGES WERE NORMAL ALSO. I THEN HEARD DEP COME ON AND TELL US TO TURN TO 050 DEGS IMMEDIATELY AND THAT THERE WAS A PLANE ON OUR R SIDE (AN RJ). WE MADE THE TURN AND I WAS STILL LOOKING FOR THE CAUSE OF THE LIGHT. WHEN I COULD FIND NO REASON I CANCELED THE LIGHT AND WAS CONTEMPLATING ON DECLARING AN EMER; BUT I DID NOT KNOW FOR WHAT REASONS. DURING OUR CLBOUT; I HAD HIM ROTATE THE SELECTOR SWITCH TO THE 'ANNUNCIATOR TEST' POS TO SEE IF A LIGHT HAD BURNED OUT. ALL THE LIGHTS TESTED NORMALLY AGAIN. I TOLD MY PARTNER I WOULD BE OFF LINE FOR A SECOND AND QUICKLY CALLED OUR COMPANY MAINT HOTLINE AND ADVISED THEM OF THE ISSUE. HE STATED THAT WE WERE DUE TO COME BACK TO THE HOME BASE ANYWAY LATER THAT DAY AND DID WE JUST WANT TO FLY THERE. I URGED HIM THAT I HAD A MASTER WARNING LIGHT GO OFF FOR NO REASON AND I WAS LNDG ASAP. I THEN TOLD HIM ZZZ4 BECAUSE IT WAS IN OUR TKOF BRIEF THAT THAT IS WHERE WE WOULD GO IF WE HAD ANY ISSUES. I HUNG UP THE PHONE AND TOLD PF WE WERE GOING TO ZZZ4 AND HE AGREED. DEP HAD GIVEN ME ANOTHER FREQ TO SWITCH TO AND TO CALL THEM WHEN WE LANDED IN ZZZ3 FOR THE LOSS OF SEPARATION INCIDENT. I THEN NOTIFIED HIM THAT WE NEED TO DSND AND LAND IN ZZZ4 IMMEDIATELY. HE ASKED THE NATURE OF THE EMER AND DID WE NEED TO DECLARE ONE. I THINK I TOLD HIM I WAS UNSURE OF THE PROB; BUT WE HAD A MASTER WARNING GO OFF FOR NO APPARENT REASON. WE WERE GIVEN VECTORS TO ZZZ4 AND DSNDED NORMALLY AND WERE ASKED A FEW TIMES IF WE NEEDED THE EQUIP STANDING BY. I DECIDED THAT ALL THE SYS APPEARED NORMAL AND WE DID NOT NEED ANY ASSISTANCE. WE HAD CONFIGURED THE AIRPLANE WITH GEAR DOWN AND FULL FLAPS IN AN EFFORT TO BURN AS MUCH FUEL TO MAKE LNDG WT. WHILE ON THE DOWNWIND LEG OF ZZZ4'S RWY 2R; THE MASTER WARNING STARTED FLASHING AGAIN WITH NO APPARENT REASON. I THEN TOLD HIM TO FORGET ABOUT GOING SLOW AND BURNING FUEL. THAT WAS THE SECOND TIME AND TO GET THIS PLANE ON THE GND FAST. A NORMAL APCH AND LNDG WAS MADE. WE TAXIED INTO FBO IN ZZZ4 AND WROTE THE DISCREPANCY UP. MECHS AT ZZZ4 TOOK THE AIRPLANE INTO THE HANGAR. I CALLED THE TRACON AND THEY TOOK DOWN MY INFO; ADDRESS; TELEPHONE NUMBER AND PLT CERTIFICATE NUMBER AND SAID THEY WOULD FORWARD THAT TO THE FSDO. WE BOTH THEN FILLED OUT AN INCIDENT RPT WITH FLT OPS AND FAXED THEM IN.