2004 Citroen C3 Oil level is decreasing with no apperant re

I have a 2004 Citroen C3 about 43000 millage
I used to change my oil periodically in its appropriate time, I have no comment on the car performance but lately the oil level is decreasing over time for example after changing the oil and traveling for about 3500 mile it lost 0.75 kilogram (mostly long distance trips on average speed of about 60 mile/hour).
I never noticed any oil leaks under my car at any time. I took it to the Citroen service center they made a test on the engine twice and said that its performance is at its best. They checked every other possible reason like the belt and found that every thing is OK. They also asked me to leave the car so they could run it in the morning and they say that they noticed a slight black smoke when thay started the car at the morning. They said that they will need to open the engine to check from inside ! Do I need to do this, as the car is still almost new how could there be a problem in the engine ! And this will extremely under value the car and will be too expensive.
What could be other possible reason for this oil level decreasing, could the oil be vaporising as it is hot here where I live and I am making mostly long trips? A friend of mine told me that it could be mixing with the water cooling system, is that true?
Thanks