Yes, my worry is was some sort of very high moisture environment (dripping garage, partial flooding, etc).

That have a bunch of hidden deep down issues.

I'd take one of those endoscope cameras and look behind rear seat, quarter panels, inside holes in frame rails.

And take real close with high quality close up pictures around inside and outside of rear window gasket. Then blow them up on computer screen to see if there are traces of leakage or rust.

Also flash pictures from under the dash looking up into it. You can see rust from moisture buried under the dash frame area. There's always some flash rust. But looking for heavy fresh rust.