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.