It seems your not understanding how and when carb ice can form. Carb Ice can form at almost any temprature given there is enough moisture in the air. Here is a nice chart to show you. http://www.kfackler.com/gtuf/carb_ice.html
So if you said it's 27ºC out thats around 81ºF (sorry the chart is in ºF) Ok so look up the 81 in ambiant temprature. To get ice to form the dew point only has to be around 38F so thats what 3C? So around 20% humidity. Chances are the dew point was higher. Take a look at the chart even at 40% humidity you well into the serious icing at gliding power. Dont let that "Serious gliding power" fool you. Its just a fancy way of saying anying under cruise power.