By default, after you first save a document, Pages will autosave it with each change that you make to it. Autosave can do this within 5 - 10 seconds for a local document. The black dot will only appear prior to an autosave, and then it is gone until changes accumulate prior to the next autosave event. If you are not making changes to the document, there will be no black dot appearing in the close button.
Elsewhere in the responses here, one can make changes in the System Preferences : General panel to disable autosave, and thus, the black dot will remain in the red close button until you manually save the document. It is easy to lapse into a trance when writing, and forget to manually save. A power outage, or Pages crash/hang could cost you a good deal of unsaved work without autosave enabled.