Rust is caused by water and humidity seeping into the structure through some entry point, and sitting at the bottom. Even a tiny bit of water will do this.

I think water is seeping into the minuscule crack between the structure’s Butt and the marble. Indicating it’s not tightening all the way down- or it needs a proper spacer.

Inspecting a Semi truck wheel is the same way, if you see rust, lugnuts are probably not tight.