The radiator hose was an initial problem with some leaking at the connection point of one of the hoses and that helped but was still getting coolant warnings around 5-7K miles so recently I finally got around to getting to the bottom of the problem once and for all.
The culprit was a small leak, very slight, on the small hose attached the main coolant hose (#1)
This is directly above the serpentine belt and was contaminating it, along with seeping oil from a crankshaft seal leak.
The replacement to #1 on the part diagram no longer includes the small hose returning to the Y adaptor. BMW must have realized that it's location and small connection was a stress point prone to failure.
The replacement hose also requires buying a cap. Here's a
pic of the current hose used when replacing the original. Long story short - after replacing this hose, sans the old vent host replaced by the cap, the coolant level has remained constant.