Documents marked "not for public release" aren't classified. They're what's called controlled unclassified information (CUI). It's anything from PII, law enforcement victim records to sensitive (but unclassified) technical manuals. There's dozens of categories if anyone cares to look at them: https://www.archives.gov/cui/registry/category-marking-list
They shouldn't be sitting out there, but it's also not a crime.