DURING TAXI OUT; THERE WAS CONFUSION AS TO WHETHER A FUEL CAP HAD BEEN LEFT OFF THE ACFT. THE COMPANY CONFIRMED THAT THE FUEL CAP WAS ON THE ACFT; SO THE FLT DEPARTED. AFTER LNDG; IT WAS DISCOVERED THAT THE FUEL CAP WAS MISSING.

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DEPARTED GATE AND TOLD TO TAXI TO RWY 36R. 1/2 WAY THERE; GND CTLR TELLS US TO CALL COMPANY. COMPANY SAYS THAT THE FUEL CAP WAS NOT PUT BACK ON ACFT; AND THAT WE SHOULD RETURN TO GATE. WHEELS UP TIME WAS ABOUT 10-15 MINS FROM CALLING COMPANY. WE ASK IF WE CAN RETURN TO THE RAMP AREA; HAVE A COUPLE OF MECHS BRING THE CAP IN QUESTION TO THE ACFT; WHILE KEEPING #1 ENG RUNNING. SO WE SIT AT SPOT 8 FOR ABOUT 5 MINS AND A TRUCK WITH 2 MEN AND A LADDER IN THE BACK COMES ROLLING UP TO OUR ACFT. NO SOONER THAN THEY ARRIVE THEY GIVE US A THUMBS UP AND WAVE GOODBYE. NOT GREAT COM; BUT TO BE SAFE WE CALL BACK INSIDE TO OPS TO MAKE SURE. THEY SAY THAT THE CAP WAS ON ALL THE TIME. WE TELL THE PAX THE TRUTH; AND TAKE OFF WITHIN 1 OR 2 MINS OF OUR RELEASE TIME. ACFT LANDED WITHOUT FURTHER INCIDENT; EXCEPT WHEN WE GET BACK TO THE GATE; OUTBOUND CREW IS TAKING OUR ACFT BACK OUT AND AFTER COMPLETING WALKAROUND THEY SAY; 'WHERE'S YOUR FUEL CAP?' SO WE ARE AT A LOSS TO EXPLAIN WHY THE 2-MAN CREW DIDN'T PICK UP THE FACT THAT WE WERE WITHOUT A FUEL CAP.