A PA28 PILOT FAILED TO SECURE THE RIGHT FUEL CAP BEFORE TAKEOFF AND NOTICED THE CAP WAS GONE AND FUEL SIPHONING OVERBOARD DURING CLIMB. HE RETURNED TO DEPARTURE AIRPORT TO LOCATE AND INSTALL THE FUEL CAP.

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I LANDED AT ZZZ ARPT AND HAD MY ACFT FUELED AT FBO. WHILE CONDUCTING THE PREFLT PRIOR TO DEP FROM ZZZ; I SUMPED THE FUEL FROM BOTH TANKS TO CHK FOR WATER. I OPENED THE R FUEL CAP TO CHK THE FUEL LEVEL AND REPLACED THE CAP BUT DID NOT FULLY LOCK IT; INTENDING TO RETURN THE SUMPED FUEL TO THE TANK. I SUMPED THE FUEL AND DECIDED TO RETURN THE FUEL SAMPLE TO THE L TANK FORGETTING TO FULLY LOCK THE R FUEL CAP. AFTER TKOF FROM RWY X AT ZZZ; AND AT 3000 FT MSL ABOUT 10 MI N OF THE ZZZ ARPT; I DISCOVERED THAT MY R FUEL CAP WAS MISSING. FUEL WAS BEING SIPHONED OUT OF THE R TANK; WHICH WAS THE TANK BEING USED AT THE TIME. I SWITCHED ON THE FUEL PUMP AND SELECTED THE L TANK USING THE FUEL SELECTOR. I THEN CALLED ZZZ TWR; RPTED MY POSITION AND THAT I WAS INBOUND FOR LNDG. ZZZ TWR CLRED ME TO LAND ON RWY X. I RAISED THE NOSE OF THE ACFT AND SLOWED THE ACFT TO 80 KTS TO MINIMIZE FUEL LOSS. ON FINAL APCH TO RWY Y; I INTENDED TO NOTIFY THE TWR THAT I HAD LOST MY FUEL CAP AND THAT THERE WAS THE POSSIBILITY IT MIGHT HAVE FALLEN OFF ON RWY X. THE FREQ WAS BUSY; SO I DECIDED TO LAND AND RPT ONCE ON THE GND. UNFORTUNATELY; I FORGOT TO DO SO. HAVING LANDED AND TAXIED TO FBO; I INFORMED THE LINEMEN ABOUT THE LOST FUEL CAP. THEY CONTACTED ZZZ GND FOR PERMISSION TO LOOK FOR THE FUEL CAP ON RWY Y. THEY LOCATED THE FUEL CAP IN THE GRASS ON THE L SIDE OF RWY X ABOUT 150 FT DOWN THE RWY. FROM THE COCKPIT; I ALWAYS CHK TO BE SURE I HAVE REPLACED THE FUEL CAPS PRIOR TO TAXI AND DID SO AFTER I HAD BEEN FUELED. HOWEVER; SINCE THE FUEL CAP WAS IN PLACE BUT NOT FULLY LOCKED; I DID NOT NOTICE THAT IT WAS NOT SECURE. I LEARNED 2 CORRECTIVE LESSONS FROM THIS MISTAKE. THE FIRST IS TO ALWAYS FULLY LOCK THE FUEL CAPS EVERY TIME THEY ARE OPENED TO PREVENT FORGETTING TO DO SO. THE SECOND IS TO INFORM THE TWR AS SOON AS PRACTICABLE ABOUT THE POSSIBILITY OF FOREIGN OBJECTS ON THE RWY. I SHOULD HAVE NOTIFIED THE TWR ON MY INITIAL CALL. IF THE FUEL CAP HAD COME TO REST ON THE RWY INSTEAD OF IN THE GRASS; THIS COULD HAVE BEEN A SAFETY ISSUE FOR OTHER DEPARTING AND/OR LNDG ACFT. I ALSO LEARNED THE RATE AT WHICH FUEL SIPHONS FROM AN OPEN TANK. FROM TKOF TO LNDG; APPROX 1 GALLON OF FUEL WAS LOST.