Depending on how cold and long you were up for the fuel in the tanks can act as a cold soak. When you descend/land the higher humidity causes frost to appear on the airframe next to the tanks, in fact you can quite nicely see the outline of the tanks sometimes which shows you where they are and how big they are if you didn't know!
I agree with Martin to an extent, most club hacks have that many dents and bumps in the leading edges that you're probably simulating icing every time you take off in them.