im not familier with the mondeo air con systems, but think there are a manual temp and an automatic temp system..
if this is an auto system then its not unusual for air con to come on automaticly with whatever heater temp is set, soon as engine is turned on as the AC is excellent for clearing screens etc
i take it the leak you see is clean water, if its coloured then its coolant..
also when you say AC is switched off do you only mean temp is set high on heater, as this wont stop the AC working automaticly but you should have some kind of indicator on the control panel to say AC is operating and be able to turn it off...as said im not familier with the set up but they all work in similar fashion.
easiest way to see if air con is operating is to look at the clutch on AC compressor, should see/hear that clutch kicking in and out with engine running