DestinationPageUrl, if not specified explicitly, will take you back to the page you requested but were not authenticated, so you were sent to the login page. Logging out will send you to the default login page, which indeed, as you correctly observed, is ~/Sitefinity. By design the Navigation control will display only these pages to which the user has granted permissions, however we're currently working on a fix with this functionality, since there seems to be a bug with permissions not applying correctly. If you want to completely hide the page from the Pages grid, I'm afraid this is not included at the moment, but thank you very much for pointing out the need for such functionality.
I hope you find the above information useful. if you need any further assistance, please write back.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items