ive only ever seen rust on a focus where its been repairs before

ie rusty wings from being replaced without having the backs sealed, rear arches starting around a blob of filled that was dumped into a scuff
im suprised yours has it without something going on

my mums 03 mp3 has 2 bits of scabbed up paint but on either door, its a very clean car otherwise, weird that its there, not had skins on either