I assume they don't brush off. There are a number of possibilities. One way to narrow down what might be going on is to make two tests. Get a few Vitamin C tablets and crush them and put the powder in a sock. Rub the sock gently on the stains and see what happens. Also, get a trichlor tablet and rub it gently on the stain and see what happens.
Vitamin C will lighten or remove the stain if the stain is metallic. The chlorine will remove it if the stain is organic. I just put a vitamin C tablet on the spot and leave it overnight.
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