I had a similar situation on my 528 but it was a cracked head. I pray for your sake this is NOT to be the case; please take notes of the following:
1. coolant leake does NOT mean you will see coolant in your driveway and/or under the car. it will escape while driving (I am assuming when pressure buildups up on a cracked head or a cooked head gasket) a good sign for this (you will see and/or smell white smoke
2. On my 528 (I was told) one of the spark plugs hole is TOO CLOSE to the oil or antifreeze lines and any over torque when replacing spark plugs may crack it-- A aluminum . This was my mechanic's justification for relacing the head on my car.
3. The head gasket itself is a very good place to search for leaks
4. Also don't rule out the heater hoses as well. Check the hose connections where the clamps are applied. Small tears or wear areas can occur from the hose expanding and contracting.
whatever may be the case I wish you the best of luck