The 12 year warentee ONLY covers rust that has caused perforation, ie a hole, the paint itself is only garenteed for a year, and general rust garentee is 6 years. All ST170's are older than 6 years now.
Rust under the door mirrrors is the first place to look at when trying to find rust on a focus, first thing i did when i got mine was to remove the mirrors, clean under them and fit sticky backed foam under them to prevent the mirrors rubbing the paint away and causing rust.