I've owned my 156 for 5 weeks now and discovered various rust traps:
On the inner wings where the brake sensor cables run there is a small plate to hold the cables , water and muck collect here and spread. If you havent had a look here check this weekend, mine didnt look too bad but would have been terminal if I hadnt caught it quickly.
Also the floor seems to have punched semi circular drain holes, mine required work on most of these, essentially filling and covering over.
The floor just behind the front wings is another area to watvh, mine required welding in this area, so the carpet and drivers seat needed to be removed.
I'm sure there is more that I havent found yet.