Hi!
I had that leak to and BMW pointed at the vanos gasket (the one against the block). They replaced that, and some o-rings for the regulating valve in behind there.
The front and belly of the engine was marinated whith oil and guck, but it didnt leak so much it would drip. And it wasnt visible from above as you say, nor on the dip stick or from under.
It was over a year ago I had it repaired and I really hope its leak free now. I've cleaned the engine twice (from under and above) since the repair and both times I've notised a small amount of oil that looked new/amber. Seems like it originates from the chain tensioning bolt area. And it's also following the head gasket around the front of the engine to a point just above the 2nd header pipe. I will have that checked in the spring. Hopefully its just lingering oil from last time, there are lots of places for oil to hide, and it hides well.
