2001 BMW 330 Engine Using Oil

My brother has a 2001 BMW 330Ci that Im trying to help him with. His car is using a quart of oil about every two weeks. He took it to the place we usually take our cars and generally they are great. They diagnosed it as a bad PCV valve. So he paid about $700 to fix that and about a month later, he had the yellow oil light come back on and sure enough, the dipstick was very low, almost to the bottom. The car is not leaking anywhere we can see and no matter where he parks or how long the car sits there, there is never any drips. Also the car isnt sending any blue smoke out the exaust that we can see. The car actually just passed smog the other day. Do you guys have any ideas?