E46 Coolant Loss-no leaks

[Karate Kid]

Hi Guys, I have a 318i E46 prefacelift with abot 245000kms. EVry 200 km's the low coolant level light comes up. I top up and thsi takes about 500ml. I have done a pressure test on he cooling system and there are no leaks to be found. The expansion tank and a few pipes were recently changed when I previoulsy had a coolnat loss. There have not been any temp excursions and the oil looks clear.
Any advice?

 

[BMW M]

Might indicate head gasket...

 

[Karate Kid]

Might indicate head gasket...

Whats the best way to check? The oil does not seem to indicate any emulsion. A mechanic suggested that the thermostat is not opening properly and therfore kicking back the water through the expansion tank cap