I bet it is coming from the hub.Could the "rust" be coming from the rotor hub itself and working it's way out.
If water gets in there, I'm sure it does, then it will forum a rust soup.
That might eat at the finish of the wheel.
The wheel and cap I don't think will have "Iron" rust.
Any place water can collect, needs a drain to get rid of it quick and allow it to dry.
I wonder if putting a fine notches in the cap might be worth it.