A FLC MEMBER OBSERVES AN ACCUMULATION OF UNDER WING ICE ON A RECENTLY PARKED B737.

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I FLEW IN FROM GRR TO ORD IN B737-200 AS FO. WE PICKED UP MODERATE MIXED ICING FROM 6000 FT AND BELOW. WE HAD AN ACFT CHANGE AT ORD. UPON ARR AT GATE; I TALKED TO THE INBOUND CREW OF THE ACFT WE WERE GOING TO PICK UP. THEY RPTED THAT THEY HAD MODERATE ICE INBOUND TO ORD. THEY WERE AT 6000 FT AND BELOW IN ICING CONDITIONS AND TOLD ME THEY PICKED UP A LOT OF ICE INBOUND AND THE ACFT WOULD PROBABLY NEED DEICE. THEY SAID THEY HAD ENG AND WING ANTI-ICE ON. THEY ALSO SAID APCH VECTORED THEM ON A 25-30 MI DOWNWIND AT 6000-4000 FT MSL. DURING PREFLT I NOTICED THE UNDERSIDE OF WING FROM APPROX 2 FT BEHIND SLAT AFT HAD A SIGNIFICANT LAYER OF ICE COATED ON IT! THE FIRST 2 FT BEHIND THE SLAT AND THE SLAT ITSELF WAS CLR OF ICE. I THEN RECEIVED GND CLRNC AND LOWERED THE FLAPS TO 25 DEGS TO INSPECT FURTHER. EACH LEADING EDGE OF THE FLAPS HAD APPROX 1 INCH OF ICE ON IT -- SIMILAR TO WING LEADING EDGE ICING. FURTHER INSPECTION I SAW THE GAP BTWN THE WING AND LEADING EDGE OF AILERON TO HAVE ICE BUILDUP ON IT. ICE BUILDUP ON LEADING EDGE OF AILERON -- JUST ON LOWER FRONT SIDE OF AILERON. THE UNHEATED TAIL LEADING EDGES ALSO HAD AN ACCUMULATION OF ICE. FLAPS 10 DEGS AT 170 KTS IS THE MINIMUM XOVER SPD ON THE B737-200 IN THIS CONFIGN; HOWEVER; 1.3 VSO IS WELL BELOW 170 KTS AT FLAPS 10 DEGS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR IS VERY CONSCIOUS ABOUT ICE ACCUMULATIONS ON ACFT AND THE DETRIMENTAL EFFECT IT CAN HAVE ON FLT PERFORMANCE. HE FEELS THERE IS NOT ENOUGH DONE TO PREVENT ICE FROM FORMING ON ACFT. HE HAD NO FLT PERFORMANCE PROBS THAT DAY WITH ICING; HE JUST WANTED TO RPT THAT ICING CAN FORM ON THE BOTTOM OF WINGS AS WELL AS ON TOP OF WINGS. HE ALSO OBSERVED ICING FORM IN THE GAP BTWN THE WING AND WING FLAPS AND AILERONS ON THE BOTTOM OF THE WING. HE WANTS ALL TO CONSCIENTIOUSLY DEICE THAT AREA.