expression, noun which may take the genitive case particle 'no'
now available; currently released; now showing
Used on notices, websites, advertisements, and listings for things currently open or released to the public, such as films, exhibitions, documents, videos, or sales offerings. It can modify a noun with の.
The new film is now showing nationwide.
The survey results are currently available on the website.
I went to see the exhibition that is currently open.
非公開 means something is kept private or not released to the public, the opposite of 公開中, which means it is currently open to public access.
A transparent compound of 公開, meaning public opening or release, plus 中, meaning being in the middle of a state or currently ongoing.