There is ONLY 2 places the oil can go, leaks or burning it. If you are not seeing oil on the ground then your engine IS burning it. You just cant see it. With todays efficient converter configurations, chances are the "blue" smoke that you would see with burning oil has been "cleaned up". I would start with replacing the Crankcase Ventilation Control Valve. You may have to buy this part from the dealer, many aftermarket parts houses don't carry it.(#11617501566 $60.87) It is located in the intake manifold near the throttle body (NOT easy to get at) Hope I was able to help
It is what I would call a difficult repair. And you can bet your BMW dealer wont be much help. You will have to have a selection of tools to use. If you have a lot of tools you may be able to tackle the job yourself. be cautious, don't force anything.