Guys that do termite inspections for existing home sales use a pointed stick to poke wood framing to look for soft spots. Paint can cover the rot and hide it well but a hard stick can detect where wood is rotten or patched poorly.
The same applies with rust on cars. A screwdriver can be used to poke at areas that you suspect. To me, your car has just simple surface rust where bare metal allowed some minor oxidation to occur.