Ford EcoBoost engine issues

[Must16]

Just got the car back from the dealer, they checked under the rocker cover with an inspection camera. No sludge, oil condition ok. As for the cooling Carried out pressure test-all ok. visual inspection -no leaks. But this reason was given for the low level. Coolant system is under high pressure which makes coolant level go down, however once you open coolant cap coolant level goes up. Any thoughts?