Just want to share a bit of knowledge about this, I've owned a 1.25 mk4 fiesta and now a mk6.5 1.6 zs. Both of these have had the problem, it's common for the plastic coolant manifold on the front of the engine to warp.

When cold/warm it doesn't leak but when the engine is hot it starts pouring out and evaporating so very hard to spot. If you have trouble with losing coolant this would be a good place to start.

Hope this helps people =]