If you use a wet rag to hold some iron out against the stain for a few minutes . . . and it's not removed or lightened, it's almost certainly not iron.
If you do the same, but using dichlor or cal hypo . . . and it's not removed or lightened, it's almost certainly not tannins from leaves, etc.
Those are the only two brown stains I know about. I have seen some brown mystery stains . . . but I never figured out what they were, only what they weren't.
Good luck,
Ben
PS. Do NOT let any of those three chemicals -- Iron Out, cal hypo, or dichlor -- come into direct contact with each other. If you do, bad things WILL happen.
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