Window Frame Rust

[JPS48]

My white 2009 which is barely used in winter and has less than 15,000 miles on it is showing rust on the inner side of almost every door window frame where they meet the door. Not very much but just enough to notice it. The welds are all fairly smooth and well painted but on closer examination seem to have developed very small hairline cracks which I assume are caused by a small amount of flexing when a door is closed. It's in these cracks the rust has appeared.

I've owned many vehicles, some for very long periods but can't recall rust on any in these locations.

Is anyone else experiencing this?