BE1900 FUEL CAP WAS MISSING AFTER THE CONCLUSION OF THE FLT.

Narrative

AFTER RECEIVING THE RELEASE FOR OUR RETURN FLT TO LAS; WE ASKED THE FUELER TO FUEL THE ACFT TO THE PLANNED FUEL OF 2800 LBS. WHEN THE FUELING PROCESS WAS COMPLETE; A THOROUGH WALKAROUND WAS COMPLETED BY THE FO; THEN THE BOARDING OF OUR SINGLE PAX COMMENCED. WE CONDUCTED THE BEFORE START CHKLIST; INDICATING THAT THE FUEL CAPS WERE INDEED IN PLACE AND APPEARED TO BE SECURELY IN PLACE. THE FLT TO LAS WAS RELATIVELY UNEVENTFUL; EXCEPT FOR OUR REQUEST FOR A HIGHER ALT DUE TO MODERATE; CONTINUOUS TURB. FUEL BAL WAS MONITORED AND FOUND TO BE EQUAL THROUGHOUT THE FLT. UPON LNDG IN LAS; THE FO CONDUCTED HIS POSTFLT EXAM OF THE ACFT AND HE AND THE FUELER SIMULTANEOUSLY FOUND THE R MAIN FUEL CAP TO BE MISSING. AFTER SEARCHING FOR A MOMENT AROUND THE ACFT; OPS WAS IMMEDIATELY NOTIFIED. THE FO ASSURED ME THAT THE R MAIN FUEL CAP WAS IN FACT THERE PRIOR TO DEP. I THEN CONTACTED THE A&P WHO DOES OUR FUELING IN ZZZ. I HAD ASKED HIM IF HE REMEMBERS ANYTHING UNIQUE ABOUT THE FUELING PROCESS; BECAUSE WE LANDED IN LAS WITHOUT A FUEL CAP. HE ADAMANTLY STATED THAT HE WAS SURE HE PLACED THE FUEL CAP ON; STATING HE HAS BEEN DOING THIS AS AN A&P FOR YRS. WHEN I ASKED HIM FURTHER; HE SAID THAT HE REMEMBERS THAT THE FUEL CAP APPEARED TO BE WORN. HE ALSO MENTIONED THAT THE FUEL CAP HAD NO CHAIN ATTACHED TO IT. HE REMEMBERED THIS BECAUSE HE TOOK THE CAP OFF TO FUEL; PLACING IT ON THE TOP OF THE WING; AND THE WIND (ASOS XA42Z WIND 320 DEGS AT 24 KTS GUSTING TO 32 KTS; 10 SM VISIBILITY; CLR) TRYING TO BLOW IT OFF MANY TIMES. I HAD ASKED HIM TO RETURN TO THE RAMP AREA; INSPECT THE TXWY AND RWY WE TOOK OFF FROM; LOOKING FOR ANY POSSIBLE EVIDENCE OF A FUEL CAP FALLING OFF PRIOR TO FLT. HE DID SO; CALLING ME BACK APPROX 1 HR LATER SAYING THAT HE HAD INSPECTED EVERY INCH OF THE ARPT; AND FOUND NO EVIDENCE. HE AGAIN STATED TO ME UNEQUIVOCALLY THAT HE REMEMBERED SECURING THE FUEL CAP.