First off the answer to your first post, there isn't one.
The Focus uses a NTC (Negative Temperature Coefficent) sensor which is between two of the spark plugs this measures the temperature of the metal itself and hence the running temp of the engine, this info is used by the ECU to determine the engines fuelling requirements and also for the cooling fans and the temperature gauge.
Now for the second:
The cooling fans will come on when the AC is switched on OR when the heat direction is set to windscreen REGARDLESS of blower position or AC setting, this is why the icon is orange coloured.
The cooling fans will only come on to cool the coolant due to insufficent air flow so moving in slow moving traffic or at a standstill for prolonged perioids of time.
Martin