Hi Jan,
There should be an unsubscribe record for this person under Marketing > Email Marketing > Unsubscribes. If you don't see anything, then perhaps that person was unsubscribed at the time of the mailing and then opted back in. Depending on how they unsubscribed you may or may not be able to see this in the Audit History (if available).
Please let me know if you this doesn't seem to be the case or if you have questions. Thanks for using the forum!