I am having difficulty finding the engineer manual for your model.
The majority of Miele fridges thermostats are located in the light compartment, where have you been looking?
As the fridge has a auto defrost setting one sensor will control when that takes place, the fan is designed to control moisture and prevent frost.
Most sensors work based on a make contact, as the condition which requires action the contact is made. So in this case as the fridge gets warm the thermistor should reduce its resistance, I would use a multi meter to read the resistance of each sensor, set the fridge to 'supper cool' and check every 30 minutes and see if the resistance is changing. After a couple of hours I would then do the opposite switch off the fridge and let it warm up and see if they increase resistance as it warms up. The sensor which isnt changing is the one at fault.