Coolant loss

I changed the distribution on a Ford focus Mk2 hatchback from 2006 1.8 Tdci. After a short ride a had to fill the tank with coolant (the mechanic told me so). After like 2000km I was checking my oil level and I saw that the coolant tank its empty. I added 0.5L more and after that no more loss. I have like 3000km now since then and nothing. Any ideas for the reason?

A lot of the new engines are designed in a way that there isn't a direct bleed for trapped air when the engine is turned off. Some manufacturers have put in a bleed screw to remove this air. Ford doesn't, when the coolant gets low or is let out for work, you have to check it for awhile. So it will take a few run cycles for the system to purge the trapped air on its own since there is no way except with a vacuum device to get it out.