I've never noticed that area as a potential corrosion point, so my guess is that it's been in an accident, your log book should say if it has, or a vin check will clarify.
There are several options
1) if the car is true and straight, get the chassis runners rewelded to the car, preferably seamed.
2) put the car on a jig and lasered true, this will enable the technician to clear the car in an engineer's report and they can weld where necessary.
3) Forget you saw it. (not really an option)
If you go for the laser option don't go to franchised garages, a good reputable independent repair specialist will be able to repair it at a minimal cost to you.
Hope this helps.
D