Why is my electric furnace warm if it isn't being used?
Could be a faulty thermostat. If you have a multimeter, check if there is any potential on the output, change the setting for a temperature, higher than the ambient and check again. If the electrical potential is the same you may need to change the thermostat. IDK, but I think the heater can't just turn on by itself- there must be a stuck relay/contactor or the thermostat is faulty.