There is no radiator fan switch on a Focus. The fans are turned on and off, slow speed (via (i)another(/i) resistor pack in the shroud) or high speed by 2 relays in the engine bay fuse box. These are operated directly by the PCM/ECU.
I don't know the precise logic involved, but usually assume that the primary measurement that triggers them is the Cylinder Head Temperature (CHT) sensor. Again, except in the ST170, there is no coolant temperature sensor.
The PCM, as well as being part of the aircon clutch switching, takes a signal of aircon on (demand). As the condenser in front of the radiator must have airflow through it for the cooling to work, the PCM switches the fans on.
So suspicion must fall initially on the CHT. That is however strange as its the sensor that is also used by the PCM/ECU to generate the signal for the 'gauge' in the dash. However, my opinion is that the 'gauge' is more of a 3 zone indicator, cool - OK - too hot, not a true temperature gauge as in earlier direct driven vehicles. So the CHT could be sending an 'OK but hot' signal, which would turn fans on, but not show on the dash.
Could be investigated further using an EOBD reader and programme to monitor PCM coolant temperature, or simply replace the CHT.....
Oh, finally, some Focus have a Ford work-around for high under bonnet temperature of an 'old fashion' fan switch cable tied to the battery tray. Its directly wired to the battery so if the temperature goes over ~90, even after the engine is off, the fans come on!!
Martin