I could be completely off here... but maybe it's gone into your coolant... maybe your head gasket has failed between a coolant duct and an oil duct, and the oil and coolant are mixing??? the coolant that goes into the oil is burning off,and the oil going into the coolant is circulating with the coolant and gradually replacing it!!????
It's late right now... so I'm probably just talking $h1t, but I drained coolant out of my 325e when I was pulling the engine out, and when the coolant settled, there was a layer of oil on top of the coolant. I was pouring oil into that car every other time I filled up with gas... granted my rings were shot, and probably my valve seals were too, and I was burning most of it, but still!, I had oil in my coolant.