Check the Boot floor. Water ingress around the rear end can lead to a perpetually damp spare wheel well, which will eventually break out in scabby rust.
Also, the rear suspension "turrets", where the bump stops fit. These can rust at the very bottom, causing the bump stop to fall out.
Seen this on cars with virtually no other rust. Best fix is to use a repair section cut from a scrap Panda, although it can be fabricated by hand.
On 100HPs and Sportings the sill covers can hide the extent of the problem until it is pretty nasty. Unfortunately taking the covers off usually requires new clips to re-fit, so people tend not to bother.
As above, they are normally fine unless they have been in an accident. Pattern wings are the biggest culprit, but any original panel which has been repaired in the past needs careful checking. Especially rear wheel arches.