Hi I'm new here and just purchased my 3rd Mercedes used and having problems, im hoping someone can point me in the direction to troubleshoot my problem,Thanks!

i any experts out there with a 2012 c250 coupe w204 1.8 turbo?

I began to loose coolant a week ago, looked at oil filler cap and had slight mayo more like a latte residue on inside of cap, everything pointed at culprit #1 blown head gasket, car runs great no overheating,no white smoke, no oil in coolant reservoir, I did a basic test with coolant filler cap off running motor and looking for exhaust bubbles Nothing no bubbles??? culprit#2 common problem on c250 oil cooling housing ahaah! well I took it to a certified Mercedes mechanic and replaced the entire oil cooling unit/housing and hoses, changed the oil and seemed to be fine during basic running. Drove car next day on my usual 45 mile drive to work with heater defroster on and looked at coolant level was 1/4 down quite allot? topped off coolant drove the car home with no heater,just AC on checked coolant level after cooled off and NO COOLANT LOSS??? but still frothy stuff on cap and dipstick. Topped off again and drove to work with heater defroster on and cooled down again 1/4 coolant loss ??? WTF??? my question does anyone know of culprit #3,4,5,6??? places coolant can enter engine? any advice would be appreciated