My 23 year old gelding does the same thing. He gets these sweaty spots on his neck and near his flank just standing out in the pasture, sometimes its excessive, of course we live in Texas and it gets HOT. I talked to our vet about it and he said that it's normal, as long as the horse is sweating he's fine, it's when he stops you have to worry.
This is just the case with my horse and I am by far no expert. Maybe some more people could give you advice or you could ask your vet like I did.
I wonder if something could be out in that area- spinal wise?
Sometimes after a horse has a chiropractic adjustment the muscles in the adjusted area will quiver & sweat for a bit. I don't know if that happens if something is out though. Strange.
I'm subbing for this one. It could be something to do with his past, because he was a rescue correct? This one has stumped me. I'd get a vet out just to check him over to make sure something isn't wrong internally, or to check if something is out on his neck as it is up there. Or it could be just a little quirk of his....