Page in offline mode on live site

If you have basic workflow turned on, any page that is unpublished or archived you can still view in Preview Mode when logged in.

Just set the "Publish To" Date to "Now" and save, this will unpublish it.

ALong with basic workflow, you can edit a page and not have it go live till you hit the "Publish" on it, the basic workflow is Edit -> Publish -> Archive, and if you make any changes to a published page it goes into edit mode and the changes are only live to you while the published version of the document is live to the public.

You can change the permissions to view the page, and then only allow access to your developers or who you deem need access. But the page will still show up in for example the site navigation if the web part is pulling it. In that case you would need to go with Trevor's approach and unpublish the page.