B737-300 RETURNS TO DEP STATION AFTER ENCOUNTERING AN APPARENT FUEL LEAK.

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WE HAD A NORMAL PREFLT EXCEPT PRIOR TO PUSH DISPATCH RECOMMENDED A RERTE FOR WX. AFTER ACARS BACK AND FORTH WE ACCEPTED THE CHANGE AND CALLED CLRNC DELIVERY. AFTER PUSHBACK WE MADE THE CHANGES IN THE FMC; NOTED PLENTY OF FUEL AND STARTED TAXIING. WE ORIGINALLY HAD 10000 LBS IN THE L WING TANK; 1980 LBS IN THE CTR TANK AND 10000 LBS IN THE R TANK WITH 21900 TOTAL INDICATED. EVERYTHING SEEMED NORMAL DURING TKOF AND CLBOUT WITH SOME R RUDDER BEING PUT IN BY THE CAPT. AT FL240 WE NOTICED A FUEL IMBAL. THE ACFT WAS SHOWING 1500 LBS LIGHTER ON THE R SIDE. THE CONVERSATION STARTED WITH THE 3 PREVIOUS WRITE-UPS ON THE R FUEL GAUGE. AFTER MONITORING THE GAUGE A LITTLE WE PULLED THE ENG FUEL LEAK CHKLIST WROTE THE FUEL LEVELS DOWN AND STARTED THE TIMER. WHILE WE MONITORED THE FUEL LEVEL CHANGES THE CAPT HAD THE FLT ATTENDANT LOOK AT THE R ENG AND TRAILING EDGE FOR FUEL SPRAY AND THE R TANK WAS AT 8260. AFTER NOTICING A CONTINUOUS STEADY INCREASE IN THE FUEL IMBAL WHILE THE FUEL FLOWS SHOWED HEAVIER USAGE ON THE #1 ENG THE FO WENT BACK FOR VISUAL INSPECTION AFTER THE FLT ATTENDANTS COULD NOT FIND ANY LEAK. THE CAPT BRIEFED THE PURSER WHILE I WAS IN THE BACK. I FOUND NOTHING. AFTER 10 MINS THE TANK LEVELS WERE 9160 IN THE L AND 7400 IN THE R. WE HAD INCREASED THE IMBAL 200 LBS IN 10 MINS. WE ALSO SHOWED FUEL USED PLUS FUEL BURNED WAS 2000 LBS LIGHTER THAN PUSHBACK FUEL. WE FELT THIS SATISFIED THE 500 LB IMBAL INCREASE IN 30 MINS AND TOLD MAINT AND DISPATCH WE WERE RETURNING TO LAND WHICH WAS THE CLOSEST SUITABLE ARPT AND IT HAD GOOD VFR WX. MAINT SAID IT SOUNDED LIKE A GOOD IDEA. WHILE CONTINUING WITH ENG FUEL LEAK AND THEN THE INFLT ENG SHUTDOWN CHKLIST WHICH DIRECTED US TO THE DRIFTDOWN CHKLIST AND NUMEROUS PERFORMANCE COMPUTATIONS; I STARTED PULLING CHKLISTS FROM THE FLT MANUAL. IN THE RUSH; LOW LIGHTING; AND DURING NUMEROUS VALID INTERRUPTIONS BY ATC; MAINT; AND THE FLT ATTENDANTS I PULLED ONLY THE FIRST PAGE OF THE INFLT ENG SHUTDOWN CHKLIST. IT APPEARED LIKE THERE WAS A COMPLETE BOX AROUND THE CHKLIST IN THE LOWER LIGHT AND THE CHKLIST ENDED WITH 'LAND AT NEAREST SUITABLE ARPT' WHICH ALSO MADE SENSE FOR THE END OF THE CHKLIST. WE COMPLETED ALL THE ABOVE CHKLIST AND I ANNOUNCED THEM COMPLETE. THEN DURING OUR BRIEFING WE COVERED FLAPS 15 DEG LNDG ON RWY 26 SE LNDG AND GAR CHKLISTS; THE T-PROC FOR RWY 26. WE DISCUSSED BLEEDS OFF OR ON AND OPTED FOR ON. WE DID EVERYTHING ON OUR MISSED PAGE OF THE CHKLIST EXCEPT FOR THE GPWS FLAP INHIBIT SWITCH WHICH WE GOT AT 400 FT WITH THE FIRST WARNING. CAPT HAD HIS BEST LNDG OF THE IDENT AND WE HAD THE FIRE TRUCKS INSPECT US BEFORE WE TAXIED TO THE GATE. THE R TANK WAS STEADY AT 7000 LBS SO WE KNEW WE NO LONGER HAD THE LEAK. WE LANDED WITH THE FUEL PERFECTLY IN BAL. I THOUGHT THE CREW WORKED GREAT TOGETHER. EVERYONE WAS PROFESSIONAL.