If there is rust where you can see it, there will surely be hidden rust elsewhere.

But that picture bothers me... those ARE 'painted' parts and hence the BMW tech saying "rust on non-painted surfaces is normal" seems odd. It is a way to say something true, but not really answer the question.

"Why is that rust there? How did the paint come off that?"

Here is a picture of a car with 200k miles:

Rear differential bushing replacement