Looked at a truck today, odd rust, thoughts

[Grit dog]

Fwiw, my blue 86 was an Oregon truck all its life. A split between the wet and dry side.
Totally rust free except under windshield and back window gaskets. Did a fair amount of patching not horrible though thankfully. But it had NO rust along the door seams like the OP truck. I’d try to determine whether that is coming from the inside or not.

 

[bucket]

Rust at the roof flange is very typical in a lot of areas of the country. IMHO, that's a very mild case of it too. I believe that @saltdog could very well be correct about the drip rails. Either that or there was a hole(s) drilled in the roof at some point for a beacon or antenna or something. Or the interior sheet metal was simply never coated very well to begin with and the rust is just from condensation between the roof layers.

 

[saltdog]

I have/had similar rust and it is/was definitely from drip rails. The inside of the roof is marginally painted at best. Any water that gets in will rust and over time, you'll see exactly this. Once I used urethane to seal mine up, problem gone. But you'll want to do all the drip rails. This is a very common problem as @bucket said, whenever the truck is stored outside for long periods of time. The drip rail seal such as it is is crappy old body seam sealer and gets leaky from age. It is very much worse in a salt air environment as @1STLS1 mentioned, but this looks pretty minor to me. De-rust/encapsulate the back side too if you can.

 

[TotalyHucked]

I'd agree with it probably being caused by the drip rails leaking. Rust up top like that is very common with Florida/coastal Carolina/coastal Alabama trucks down here thanks to the salt air.