Coolant loss

[Harri]

Any ideas on cause of coolant loss - expansion tank and cap been replaced; pressure test no leaks; no spill under car: heater works (but not great) : not overheating and no other obvious problem - 2 garages now stumped as to what it could be. 

One suggestion was intercooler but 2nd garage said unlikely and at 1000£ replacement, I'd just like to know for sure the right thing gets fixed! 2014 A3 tdi 2litre. Having cam belt and water pump replaced in next few weeks also.