Anything that's undisclosed is deliberately hidden or withheld. If you put your piggy bank in an undisclosed location, you're definitely not telling your little brother where it is.
To disclose is to reveal something that was once a secret, so describing something as undisclosed means you're not doing that, but instead it's remaining a secret. Anything you choose to keep private is undisclosed, like your undisclosed reasons for turning down an invitation to go camping (you're scared of the dark), or the undisclosed grade you got on that one math test!