Possible head gasket issue

I had my head gasket done 2 and a half years ago, but have noticed during the last 3 months the water level dropping and was having to put approx 70cl of water in every two weeks. There is no creamy gunge at all in expansion tank or on the oil filler cap, last week in put in a bottle of k-seal and topped up the water level and so far it has not dropped at all.

My car went in for a service yesterday and an external pressure test was done at 10lbs and there was no leaks from any hoses but the pressure dropped to almost zero, not sure what to do next any advice would be appreciated. Is it possible the k-seal has worked and hence no water loss or am i clutching at straws.

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