It appears that this plugin prevents a 404 error page from being displayed to the user.
Once the plugin is activated, navigating to a non-existent URL will result in a blank page (no content, no markup) being returned to the browser. Deactivating the plugin corrects this issue and a proper 404 page is displayed.
Of course, the problem is I want the plugin active to protect my uploads directory but I also want proper 404 pages to be displayed to the user outside of the uploads directory.
Jeff
http://wordpress.org/extend/plugins/private-files-for-social-privacy/