Wis. Stat. § 171.33: Acquiring title to undocumented property.

Property in the possession of a museum or archives which the museum or archives has reason to believe is on loan and for which the museum or archives does not know the owner or have any reasonable means of determining the owner becomes the property of the museum or archives if no person has claimed the property within 7 years after the museum or archives took possession of the property. The museum or archives becomes the owner of the property on the day after the 7-year period ends, free from all claims.