Perkin, non-environmental ice or NEI is quite common on the B737-800 and is caused by cold soaking of the wing during flight. It appears as a thin layer of frost on the upper surface of the wing and is common around the engine pylon and wing root. It can (in extreme cases) still be noted on the wing after landing in temperatures up to +18 degrees C!! This phenomenon is also common on other 737 series aircraft but normally only builds up on the under surface of the wing and not on the upper surface like the 800/900.
Usually if the ambient temperature is above freezing the layer will melt off itself. This rate of melting is greatly increased with the addition of warm fuel. Once melted the surface of the wing can be covered in condensation that can have the appearance of ice. Crews will actually go up a ladder and feel the upper surface to ensure that no ice exists (it it is suspected).
In most European countries the uppersurface of the wing must be clear of ice for departure. For this reason, if the NEI does not melt of its own accord then the wing must be de-iced (but not anti-iced). However, the US FAA has approved the B737-800/900 to depart with NEI so long as it is only formed within a black box painted on the uppersurface of the wing.
Hope this helps.