GMT 800 cab corners/rockers rust?

[Grit dog]

So I’m sure many of you are familiar with these trucks.
Looking at a cat eye reg cab pickup. ( Not in person yet, it’s 9 hours away in MT).
Been a MT truck it’s whole life but it’s an original 50k mile grampa truck that by all accounts is absolutely pristine. Undercarriage pics look beautiful. Looks like it never even went on a muddy road. Paint on the fenders not beat up from rocks n such. Got a history of a grampas summer driver with round town trips in the winter and lotsa car washes (whatever).
But the bottom corners of the cab corners/rockers is actually rusting through.
Kinda surprised.
I know rust happens, but this thing is mint otherwise.
Drainage issue? Traps water? Can corners made of recycled soup cans?
If you saw this truck it wouldn’t make sense.

 

[TotalyHucked]

I have yet to look at one real close that had rust like that but it's certainly a common issue in the GMT800s. Cab corners and rockers both like to rust through, even if the truck is mostly cared for. I know the rockers have some drains that can get clogged and I'd assume that's why they rot but I'm not sure on the cab corners. I've had 7 of these trucks but all have been completely rust free so I never had a reason to look close. Hopefully someone else has.