Mikkelsen was last seen in Ribe, Denmark on September 1, 1990. when he rented a car in Aarhus, Denmark to go to the South. The car, which should have been turned in to the car rental agency in Zurich, was found by Italian police around the middle of the month in a service area north of Fiorenze, but there were no traces of Mikkelsen. He has never been seen again.