I just figure out. I've seen this before. Sometimes some grease residue is on the coin and it alters the color that radiates out from that area. It is a stain, but not from an external source, but from the grease on the coin. Did you find in this in a OBW roll? That would explain why it was not smeared off the coin.
(The last image made me think of a cheerio.) Then the solution was recalled to me.