> What are they hiding

the privacy of prisoners maybe. A prison cell is not a public place (nor the hallways either) so there is an actual expectation of privacy there.

Lack of access to the public doesn't necessarily mean there's an expectation of privacy. Criminals in jail typically have far less expectation of privacy in the US. That's been upheld by SCOTUS. Guards are (mostly) free to enter spaces and view prisoners at any time.