I'd suggest it is caused by the hub starting to rust, they go like the hub in the attached picture. The material is an untreated steel, so will rust if damp, wet or sweats.
Picture of my 535i when I changed my wheels to the winter set. All BMW cars I've had are like this in a short while. Water can get in, and it stays inside the 'hub to wheel' centre space. It is one of the reasons wheels will seize on the hub spigots, as the damp doesn't help mixed material interactions either. Hence why we need anti-seize in most climates.