The classic carb heat scenario is repeatedly increasing throttle to make up for increasing amounts of ice. This is a form of painting yourself into a corner
I
never just add throttle if I seem to be losing rpms, airspeed and/or altitude without first checking for carb ice.
Cruising close to a ceiling puts you into a situation where the temperature and dewpoint spread is close and carb ice is more likely. The first symptom can be a small loss of altitude from a loss of a couple dozen revs on a fixed pitch prob -- or MP on c/s.