[archwerks]

The caliper would be visible thru the wheel spokes. It should NOT be even if roads are heavily salted.
Previous owner must have used something caustic to clean off brake dust, or else BMW have a problem ! if their German car cannot withstand European road salt for 2 winters! (X6 came out in '08, so none r more than 2 yrs. old)
Previous owner must have had a boat or a house by the beach, or close to salt water.
BMW dealer should have replace those calipers as part pf "reconditioning " of preowned vehicles.
Rust to metal is like skin cancer for humans!
I'd thoroughly Examine the undercarriage of that car. There will be rust elsewhere.
Totally UNACCEPTABLE in my book. Somethings' not run properly @ the preowned dept. of that dealer, Somebody ought to get fired or demoted!

Just imagine a 2 year old BMW x6 coming to a stop @ a red lite only to reveal RUST behind the wheel spokes !!
Sure would be good advertising for MB, Porsche, Audi, Lexus, Infinity.
Reminds me of Jaguar a few years ago when I saw rust on their showroom display! I was out of there in 1 minute flat !
Did they think I'd buy their cars if their showroom display have RUST anywhere??
Do they think people r blind ??