Former government officials and contractors have been known to retain papers containing classified national security information and to eventually donate them to private archives. Often, it is not until these records are formally processed that archivists realize a collection contains classified information. If an archives or a library has not received Federal approval to store classified materials, continuing to store the records in an unapproved area could be endangering national security. In these instances, the institution should contact the Information Security Oversight Office (ISOO) at the National Archives and arrange for these records to be securely stored. ISOO will maintain temporary custody of the records through the declassification process.
https://www.archives.gov/isoo/faqs#how-does-cnss-end-up-in-private-collections