I work in the games industry, and have extensive knowledge of NDAs that may or may not be attributed to unionization efforts. If you find a random disk somewhere and the contents are confidential, you are not subject to the NDA. They can ask, and even sue, for the property back but it's not a guarantee that they'll win. There are cases like this that have gone both ways.

The owner of the disk could have anonymously leaked the contents, sold the disk, or anything they wanted with it. It doesn't matter because in general, if the disk was legally acquired then it's their property to do with as they please. Leaking the contents is the only legally questionable thing that the owner could have done with the code.