Coolant Leak with a mix of Engine Oil

The other day while driving on my 2002 Cl500, my check engine oil light came on. Stopped into the Gas Station next day to check the engine oil level to my shock it was empty. The mechanic checked the car but there was no oil leak at all.

A week later while driving, I notice smoke coming out of my ac vents, pulled over and popped the hood. The small pie that goes into the expansion tank (for the engine coolant) had come out and on the side was this coffee colored cream, that looked like engine coolant mixed with engine oil.

I drove to the service station, they said it could be the gear oil mixing with the coolant ( dont really trust them as they are not Merc technicians). Sent the car to a specialist, they are suspecting the head gasket, but want to be sure. The checked the engine oil cooler but havent found the problem.

There were no symptoms, neither did the temperature gauge rise at all, it was at 90 (considering I live in the Middle East) they said this is normal.

Could you please let me know if I should be worried, and what should I tell the technicians to check for.

Regards,
Oliver Mathews