SUSPECTED FUEL LEAK NOTED ON WALK AROUND. MAINTENANCE INSPECTED AND FOUND NO LEAK.

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DURING MORNING ORIGINATING PREFLT INSPECTION SOME OIL WAS NOTED ON BACK OF ENG NACELLE NEAR BREATHER TUBE (NORMAL FOR THIS ACFT). NOTHING ON GND OR VISIBLY LEAKING. SOME DAMP SPOTS WERE NOTED ON LEFT MAIN GEAR TIRES AND IN FRONT OF TIRES ON GND; AGAIN NO LEAKS DETECTED. AFTER ENG RUN UP AND REPOSITIONING ON GATE; THERE WERE A FEW DAMP SPOTS NOTED ON LEFT MAIN TIRES. I CONSULTED BY PHONE WITH DISPATCHER AND MAINT COORD AND DESCRIBED WHAT I OBSERVED. I ASKED WHEN THE #2 ENG WAS LAST SERVICED. THE ANSWER WAS 'THAT INFO IS UNAVAILABLE AT THIS LOCATION.' I ASKED FOR A MECH TO COME AND CHK OIL QUANTITY AND OBSERVATIONS. WHEN A MECH (NOT COMPANY EMPLOYEE) ARRIVED HE PARTIALLY UNCOWLED ENG AND CHKED OIL QUANTITY AND FOR FLUID LEAKS. ALL APPEARED NORMAL. HE SAID FLUID ON TIRE WAS FUEL NOT OIL. THERE WAS A VERY WEAK (DILUTED) FUEL SMELL TO FLUID WHEN SMELLED. THERE WAS NO VISIBLE LEAK. THE WHEEL WELL WAS DIRTY. THE MECH CONSULTED WITH OUR MAINT COORDINATOR AND IT WAS DECIDED THERE WAS 'NO PROB.' OUR MAINT COORDINATOR SAID HE'D NOTIFY OUR MECH IN DCA TO CHK THINGS OUT IF I HAD NO PROBS WITH TAKING THE ACFT. THE ON-SITE MECH ASKED IF I WROTE UP THE OBSERVATION; WHICH I HAD NOT AT THAT POINT. I TOLD HIM I'D FEEL MORE COMFORTABLE WITH AN ENG RUN UP/LEAK CHK AND HE AGREED. WE DELAYED BOARDING AND ACCOMPLISHED A RUN UP. NO PROBS WERE OBSERVED AND NO WRITE UP WAS MADE. PAX BOARDING WAS ACCOMPLISHED AND TAXIOUT WAS UNEVENTFUL. WHEN WE WERE THE NEXT ACFT TO DEPART (#1 FOR TKOF) THE TWR INFORMED US THAT A VEH FROM OPS WAS BEHIND US AND WANTED TO DISCUSS 'LEAKING FUEL;' AND TO CONTACT HIM ON GND CTL FREQ. THE VEH CLAIMED 'FUEL' WAS OBSERVED LEAKING DURING TAXI OFF THE GATE. I SWITCHED TO COMPANY TO GET AN EXPLANATION FROM THEM AND THEY SAID; 'ARPT OPS WAS CONCERNED.' DURING THE TIME I WAS ON COMPANY FREQ FURTHER DISCUSSION BTWN F/O AND VEH OCCURRED ON GND CTL FREQ. MENTION WAS MADE OF FINDING A 'NUT' ON THE TXWY THAT COULD HAVE COME FROM OUR ACFT. THIS NUT WAS DESCRIBED AS DIRTY WITH COARSE THREAD AND PACKED WITH GREASE. IT WAS NOT SEEN COMING FROM OUR ACFT NOR WAS ANY FLUID VISIBLY LEAKING DURING TAXIOUT AND WHILE CURRENTLY STATIONARY. I VISIBLY LOOKED OUT OF OPEN DV WINDOW AT AREA IN QUESTION AND NOTHING UNUSUAL WAS NOTED. BASED UPON ALL PREVIOUS ACTIONS AND OBSERVATIONS; ALL PRUDENT PRECAUTIONS WERE TAKEN TO ASSURE SAFETY OF FLT. DEP WAS MADE. FLT WAS ROUTINE AND FURTHER INSPECTION UPON ARR IN DCA SHOWED NO PROBS WITH ACFT. THE ACFT CONTINUED IN REVENUE SVC WITH ANOTHER CREW AFTER THOROUGH INSPECTION BY COMPANY MECH AND CONSULTATION WITH COMPANY DISPATCH AND MAINT COORDINATOR. THE ALLEGED NUT WAS COMATED TO OUR MAINT FAC THE SAME DAY AND AFTER EXAMINATION WAS DETERMINED TO BE A BOLT AND NOT AN ACFT PART. IT WAS 1/4' DIAMETER WITH COARSE THREAD--APPROX 1' LONG WITH A SQUARE HEAD.