Many owners spend lots of time cleaning and polishing their cars but never clean inside the lip in wheel arches. Next time your at a car meet just walk around the cars and run your fingers along the lip and feel all that mud! The XF rear arches hold tons of wet or damp road mud in there, and its slowly rotting the car away from the inside out - when you see a rust bubble its too late, as the rust has now enetrated from the inside of the wheel arch to outside of the body; filling and repainting will often only be a temporary fix. So the lip must be kept clean and washed each time you wash the car, and also treated with a good under seal product. As Jon says, the rust will attack where the spot welds are, and spread from there. Rust spots here are common on the early XFs now, and Ive see some minor evidence on 2 front arches too. Other bad places are door bottoms on inside behind rubber surround.
Also worth noting here that these cars have a lot of bare metal underneath, not treated or painted at factory. Im talking nuts and bolts, brackets and mounts, especially on suspension parts and things attached to car body. If you crawl under youll see therse are probably covered in rust. Worth getting the wire brush on these and painting over with a good black paint like Hammerite