I found these stains on vinyl siding and don’t know what may be causing it
It appears to be a repeating pattern in that it occurs about every 16 inches. But I don’t know how the stud spacing would make such marks.
This is the south facing wall.
It has not rained recently
It was cold but sunny , 9:30 am
1994 construction.
Have you seen anything like this before?
Any thoughts on the cause?
Look up oxidation caused by the nail heads under the siding getting hot. Here’s a link to a video with an explanation about it as well.
My neighbor had his house power washed and, as it dried, the dirt from water getting under the siding came out of the siding drain holes. It looked like that.
I have to agree with Junior. Too uniform and appears from coming from underneath.
It is a very uniform pattern. I would think something is happening with the fasteners. Got me as to what it is.
Good possibility there Larry! 
Here is a past post about it as well. Few different thoughts.
https://forum.nachi.org/t/nails-showing-through-on-vinyl-siding/119741
I done a roofing job on a house that had same kind of marks on the south side siding only.
Are the stains under the weep holes in the siding?
I agree with Jr, I’ve seen this in Calgary
The vertical rows of marks I seen, were about 16 inches apart as well. We found it strange it was only the south side.
I’ll go back and get my radon machine later and when I’m there I can go up on the roof and check out the siding again.
I’ll look at the location of weep holes (Scott and Larry) and I’ll stick my hands behind the overlap of siding and see if the nail heads match up with locations of the spots (Junior).
There is a layer of house wrap behind the vinyl (I check this on every home with vinyl siding) in the places I checked, but I didn’t check this particular spot. It could be missing in this area.
I let you guys know if I can find any other useful info.
Thanks for the quick responses.
Push on the siding. You should be able to feel the nails under the siding.
Unless that nail is just past the end of the piece of siding underneath, and touching the overlapped piece?
I agree with Mr Kage, dirt/chemical wash stains. Nail oxidation would be round spots.
There are some round spots.
I have one of those, but not in my home inspection kit. I keep it in my insulation rig. I wouldn’t dare unzip a piece of siding on a seller’s house. 
