Has anyone seen this on vinyl siding? Looks like ghosting, which typically is an interior issue. This home is 2 x 6 construction with drywall, Kraft faced fiberglass isulation, and OSB sheathing. I don’t know if there is fanfold insulation behind the vinyl. It’s original construction from 1991. Located in northern MN with furnace and no AC.
What is this stain? Is it an important concern?
Based on the spacing and uniformity, it appears to some type of issues with siding fasteners. Possibly rusting or corrosion?
This is a good thread on this:
Good follow up Les! 
The studs are telegraphing (thermal bridging) through the siding. The difference between the stud temps and the voids between the studs. The studs are usually warmer than the voids, which causes the organic growth to adhere to those areas more easily.
Ultimately, there could be a few possibilities. It could be as simple as thermal bridging and now just needs cleaning or a more complex issue or problem. Back in the 90’s, they did not always put a weather barrier behind the vinyl so if water is getting in there, latent damage is possible. You might consider elevating this if it is inconclusive.
looks like mildew from northern ohio…
This is what you can find behind 40 year old vinyl siding, all types of insects.
Thanks all! You’re the best.
I don’t see a lot of vinyl here, believe it or not. Never installed or repaired it either, in 35+ years of carpentry…
