I have a ford focus 2014 and it seems to be leaking coolant only when the engine is under pressure and being driven.  It has had a new coolant bottle, water pump and temp sensor  and also been checked for any signs that the head gasket has gone.  The mechanic I use has also done a pressure test but not coming up with any answers,  he is baffled and does not know what way to go now, its obviously costing me  money putting new parts on the car that are not fixing the problem.   Hope someone can help or come up with any suggestions.

Is your footwell dry? Trying to rule out an internal heater matrix leak which would certainly cause a coolant drop...

Try to think about it procedurally. Start at the coolant bottle and work your round following each pipe in turn, a decent quality torch will help.

Look for crystals that may have formed on unions, that will be antifreeze left over when the water evaporates. I use pink anti freeze so the crystals would have a pink colour on mine for example. It could simply be a small leak where a pipe joins.

On my old Fiesta the pressure release in the coolant bottle lid failed meaning it let steam out far sooner than it should. Over time the steam wasn't noticeable but the coolant did drop. However that was very slowly over a period of time.

i had a leak I couldn't find. I put radweld in. it cured it for a while and then leaked again, but this time I could see where the leak was due to the red staining from the radweld. 

it was a slight leak along bottom of thermostat cover. that was 1.6tdci engine. radweld could be considered a bit of a bodge but at least the colour helped me.

If you think at all it's a 'rain water' leak, the smartest diagnostic I ever saw was on Wheeler Dealers when working on an Aston, he placed a smoke machine in the car interior, and on closing the door, could easily see the smoke creeping out of a certain bit of the door seal. Do you have a disco smoke machine in the attic? 😂

EDIT  ignore above LOL Wrong post in thread, never have more than one window open at a a time, lesson learned!!