Place a large sheet of cardboard under the car to show up the location of any leaks.
My Duratec started leaking slightly around the rear of the engine (only noticed during MOT from a coolant stain on the gearbox bellhousing) and it turned out to be a faulty Coolant Thermostat Housing pipe that had developed a minute crack in the moulding.
This is a plastic manifold with several pipes attached and sits at the back of the cylinder head under the coil pack on the Duratec. It is a well known issue in the world of Ford Focus, etc.
I replaced it with a Quinton Hazel part from ebay for around £30.
Appreciate, your GDI engine may not be the same though.
