FLC OF AN AIR TAXI JET RETURNED AND LANDED AFTER THE TWR ADVISED OF FUEL OBSERVED COMING FROM THE ACFT DURING TKOF.

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GND CREW REQUESTED TO FUEL TWO (2) ASAP FALCON 20 ACFT AT THE SAME TIME. DUE TO TIME PRESSURES; FUEL CAP WAS NOT TIGHTENED PROPERLY ON ACFT AND TWR OBSERVED FUEL STREAMING OFF THE WING (PRESSURIZED FUEL SYS) AFTER TKOF. ACFT WAS IN IMC; WAS CLRED IMMEDIATELY FOR AN ILS APCH AND LANDED SAFELY. FUEL CAP WAS FOUND MISSING DURING POST FLT INSPECTION AND LATER FOUND NEAR THE RWY. ADDITIONAL TRAINING HAS BEEN INSTITUTED RELATIVE TO PROPER FUELING PROCS. CREW RPTED THAT THE WALKAROUND INSPECTION SHOWED THAT FUEL CAPS WERE ON AND TABS WERE RECESSED WITH TANKS INDICATING FULL OR NEARLY FULL AT ABOUT 7000 LBS. BECAUSE THE FREIGHT WAS NOT COMPLETELY LOADED; THE CREW WENT BACK INSIDE. THE LINE CREW MISTAKENLY ATTEMPTED TO REFUEL THIS ACFT AND THEN PUSHED THE FUEL CAP TAB DOWN. THE LINE SUPVR STATES THAT THE TAB CAN BE NEAR THE FLUSH POS IN A VISUAL INSPECTION BUT; IF THE CAP IS SLIGHTLY CANTED THEN IT WILL EITHER LEAK FUEL OR PUSH THE CAP OFF THE SEAT. THE CAP WAS FOUND AWAY FROM THE RAMP; NEAR THE TKOF RWY BY THE ARPT AUTH AND RETURNED TO FBO.