flyingstone
Carb ice is most likely under idle power settings (closed throttle valve) and high airflow through the carb (flying speed). It is least likely at cruise or higher power settings (throttle valave almost fully open). A cruise ice check only confirms that ice has not formed under those conditions, not the conditions of idle power. If you start a PFL at 2000 ft the engine could be at idle for as long as 3 miniutes which is plenty of time for ice to form. Furhter more since carb heat comes from the heat of the exhaust stack, you will get an instant blast of very hot air if carb heat is applied in cruise, but much less hot air if the engine has been at idle for an appreciable lengh of time, which is why the engine should get a good shot of high RPM operation when clearing the engine.