ACFT RETURNS TO DEICING PAD AFTER DEADHEADING PLT; A PAX; RPTS ICE ON TOP OF WING.

Narrative

I WAS THE SO ON A WDB FROM DEN TO IAH. THE WX IN DEN WAS 36 DEGS F WITH VERY LIGHT PRECIPITATION AND 1600 FT CEILING. AFTER ARR FROM LAX AND ON PREFLT WALKAROUND OUTSIDE THE ACFT AND ALSO VIEWING THE WINGS AND HORIZ STABILIZER FROM INSIDE THE ACFT; I FOUND NO ICE; SNOW; OR FROST ON WINGS; FLT CTLS; LEADING EDGE DEVICES; HORIZ STABILIZER OR ANY OTHER PART OF THE ACFT AS THE TEMP WAS ABOVE FREEZING. AFTER A 45 MIN TURNAROUND FROM GATE AND TAXI TO RWY 35L AND TAKING THE RWY FOR DEP; CALL FROM A FLT ATTENDANT IN THE REAR OF THE ACFT STATED A DEADHEADING PLT SAW FROST FORMING ON TOP OF THE WING. THE CAPT THEN TAXIED THE ACFT CLR OF RWY 35L TO THE DEICE PAD WHERE THE WINGS WERE DEICED. I WENT BACK TO THE CABIN TO VIEW THE WINGS AND RETURNED TO THE COCKPIT TO RPT TO THE CAPT THAT THERE WAS NO FROST VISIBLE. THE CAPT THEN TAXIED THE ACFT BACK TO RWY 35L AND WE DEPARTED FOR HOUSTON. SUPPLEMENTAL INFO FROM ACN 206221: THERE WAS SOME FROST OBSERVED ON THE WING; WHICH HAPPENED AFTER REFUELING; WHICH COULD EASILY BE MISTAKEN FOR ICE; WHICH I THINK HAPPENED. IN MY JUDGEMENT; THE AIRPLANE AND CTL SURFACES WERE CLR OF ANY ICE AND SAFE TO FLY; BUT BECAUSE AN INEXPERIENCED PLT MADE THE COMMENT ABOUT NEEDING DEICING IN THE COMPANY OF PAX; I DECIDED THE PRUDENT THING TO DO WAS TO DEICE.