It's obvioulsly leaking water from the loop system somewhere because you keep adding water to it each day. How do you drain the system down? Could it be that valve which is slightly open?
If it only loses water when the system is cold, it could be a slow leak in a pipe joint somewhere. When hot, the leak may seal up.
Edit - this post is very similar to yours and suggests it could be the expansion vessel which needs charging up with a foot pump. The OP got the code wrong and corrects it to F75 later on.
Vaillant Combi condesing boiler losing pressure daily no sign of leak - MoneySavingExpert.com Forums