ICE FORMED ON A B737-900'S UPPER AFT WING DURING A 1 HR TURNAROUND PERIOD.

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WX AT ANC WAS VERY GOOD. HIGH OVCST OF AROUND 12000-14000 FT. TEMP AT -2 DEGS C. ARR INTO ANC RESULTED IN NO AIRFRAME ICING WHATSOEVER. RWY FOR ALL PRACTICAL PURPOSES WAS BARE AND DRY. FO MADE A GREAT LNDG AND USED NORMAL REVERSE. SPD BRAKE AND FLAPS STOWED WHILE STILL ON SNOW-FREE TXWY. USED WING AND ENG ANTI-ICE ON TAXI IN JUST IN CASE. THERE WAS SOME SNOW ON THE RAMP; BUT NOT MUCH. FO PERFORMED WALKAROUND AT ANC. NO ICE NOTICED ON PLANE. FROST ON UNDERSIDE ON WING INBOARD OF ENG WAS MINIMAL AND WITHIN LIMITS OF FLT HANDBOOK. (IT TELLS WHEN YOU SHOULD CONSIDER BEING CONCERNED ABOUT TOP OF WING; AND WE WERE NOT TO THIS POINT.) SO EVERYTHING IN MY MIND IS THAT THE AIRFRAME IS FREE OF ANY ICE. MUCH TO MY SURPRISE; ON TAXI OUT WAS A CALL FROM THE FLT ATTENDANT. SHE SAID A PAX SAID THAT WE HAD A LITTLE BIT OF ICE ON THE BACK OF OUR WING BY THE FLAP AND ENG PYLON. THIS IS OUTSIDE THE FUEL TANK AREA AND KIND OF RULES OUT CONDITIONAL ICING FROM COLD SOAKED FUEL. SINCE WE WERE JUST A COUPLE HUNDRED YARDS FROM THE GATE; I DECIDED TO RETURN FOR A DEICING. THE MECHS SEEMED A LITTLE SURPRISED AT THIS REQUEST (I THINK IT MAY HAVE BEEN THEIR FIRST OF THE NIGHT); BUT SOON ARRIVED AND HAD US ON OUR WAY. I WOULD NEVER -- AND I MEAN NEVER -- KNOWINGLY TRY TO TAKE OFF WITH ICE ON THE PLANE. IN MY 20 YRS OF ALASKA FLYING; I THOUGHT ALL OF THE CONDITIONS WE MET AND PROCS WE FOLLOWED ASSURED US THAT WE HAD NO ICE. WE WERE ON THE GND FOR CLOSE TO AN HR. THE FROST/ICE MAY HAVE FORMED AFTER THE WALKAROUND. I AM NOT SURE. IF THIS LITTLE BIT OF ICE WAS THERE; IT WOULD HAVE BEEN HARD TO DETECT ON A WALKAROUND. ALL I CAN CONCLUDE IS THAT EVEN THOUGH THINGS SEEMED PERFECT; A QUICK LOOK FROM INSIDE THE ACFT BTWN LEGS WOULD BE A GOOD POLICY.