A B737-300 ON TAXI OUT AT STL WERE INFORMED BY A PLT PAX THAT FROST SEEMED TO BE PRESENT ON THE WING.

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THE CREW ARRIVED AT ACFT AT XA00 FOR THE INITIAL LAUNCH OF FLT FROM STL TO BWI. EXTERNAL GND PWR AND HEAT WERE CONNECTED AND OPERATING. WX CONDITIONS CONSISTED OF CLR SKIES; NO VISIBLE MOISTURE IN THE AIR OR ON THE GND; AND A TEMP OF -9 DEGS C. THE ACFT APPEARED TO BE CLR OF CONTAMINATION ON ITS OUTER SURFACES AND PREFLT WAS ACCOMPLISHED; ETC. PUSH FROM GATE WAS ON TIME. AS WE BEGAN TAXI; THE FLT ATTENDANT INDICATED A CUSTOMER IN THE CABIN (WHICH WAS A PLT) WAS CONCERNED WITH SOME CONTAMINATION ON THE WING. WE STOPPED ON THE RAMP AND THE FO WENT TO THE CABIN AND DETERMINED THAT THERE DID APPEAR TO BE A PATCHY AREA OF THIN HOAR FROST IN THE FLAP AREA. APPARENTLY; THE GND HEATER HAD DISSIPATED ANY CONTAMINATION ON THE FUSELAGE AND NO CONTAMINATION WAS DETECTED ON PREFLT. HOWEVER; ONCE DIRECTION WAS CHANGED DURING TAXI; THE SUNLIGHT HIGHLIGHTED SOME CONTAMINATION. WE REQUESTED DEICING WHICH WAS PERFORMED AND THEN CONDUCTED A VISUAL INSPECTION OF THE ACFT TO ENSURE NO CONTAMINATION AND THE CUSTOMER'S SATISFACTION. WE WERE COGNIZANT OF SURFACE CONTAMINATION ON ORIGINATORS IN CLR WX; HAVING DETECTED THE NEED FOR ADDITIONAL DEICING AND REQUESTING SUCH ON THE PREVIOUS MORNING OF A DIFFERENT LOCATION. HOWEVER; IN THIS EVENT THE LACK OF READILY EVIDENT CONTAMINATION MAY HAVE LED TO THE BELIEF THAT THE ACFT HAD BEEN PREVIOUSLY DEICED SIMILAR TO OTHER STATIONS. POSSIBLY AN INITIALIZATION BLOCK ON EXISTING GATE AGENT PAPERWORK WOULD NOT INCREASE WORKLOAD; BUT ENSURE COORD THAT DEICING PROCS WERE CONSIDERED/UNDERSTOOD/COMPLETED FOR APPROPRIATE WX CONDITIONS. HOWEVER; I UNDERSTAND THAT THE PIC HAS ULTIMATE RESPONSIBILITY FOR DETERMINING THE ACFT IS CLEAN AND SAFE FOR FLT.