If pulled out of circulation (which it looks like), perhaps someone glued it to stick to something for a presentation of some type, a kid project, etc. Who knows but yeah, I wasn't going to guess glue but the other two that pointed it out, totally makes sense with the circular ring I see from the pics.
It almost looks like a circular impression. If there is no depth to it, then I agree with adhesive. If there is some depth, then it's something circular pressed onto the coin (post mint).
