What was found
FUEL CAP ON THE WING INCORRECTLY INSTALLED AFTER REFUELING, FUEL CAP EVENTUALLY DEPARTED THE ACFT INFLIGHT. CAP WAS CHECKED DURING THE PREFLIGHT, AFTER REFUELING, AND THE CAP PLACEMENT WAS CLEARLY FLUSH WITH THE WING, AND THE "HANDLE" WAS FOLDED DOWN AND FLUSH. LANDED IMMEDIATELY WITH MINIMAL FUEL LOSS.