Agree, appears to be staining--perhaps brought about by partial emersion in a liquid, long since died. Maybe a cupholder coin. A long soak on 100% acetone might reduce the stain somewhat (might...).
Frogs got it right. The glue might not still be on the coin, but it was there at some time. The area under the glue was protected from the staining that the rest of the coin suffered.
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