1.6 Ecoboost 182bhp coolant issue

[David e]

Hi people I am new on here just a quick question hopefully yous can help me out .

I have a 2012 1.6l Ecoboost 182bhp Ford focus 

It is leaking  coolant but only after a few hours of driving I could drive it for 30-40 miniutes and it won't leak at at all 

After a long drive I leave the car over night and it will go from the max mark to the minimum mark on the reservoir.

Checked and leak seems to be from back of the engine but cannot find exactly where or what pipe it is comming from .

The leak only appears when it is after a long drive 2-3 hours and the driveway is full of coolant 

I done some stripping and pressure tested it and it still isn't showing leaks .

If any of you people ever had the same problem could you let me no

I have some some research and I think it could be from the de gas overflow pipe .

Many thanks