Drupal website developers and administrators benefit from its built-in feature to periodically check if any of the enabled modules and themes are out of date. Drupal performs this check daily or weekly, when its cron job is run, automatically. If there are any modules or themes in need of an update, then on the admin area's modules or themes "List" page, you will see the notification:
There are updates available for one or more of your modules or themes. To ensure the proper functioning of your site, you should update as soon as possible. See the available updates page for more information and to install your missing updates.
(Drupal version 7 is used here for the exact messages and paths.) The phrase "available updates" is linked to the "Update" page (admin/reports/updates/update).
In most cases, that page will indicate what modules or themes are out of date. But occasionally, no such modules or themes are listed. This can be confusing, and results when the module or theme is no longer supported. Thus, Drupal considers it to be in need of updating, but no updates are available.
You can confirm this by going to the updates "List" page (admin/reports/updates), where you will see the module or theme listed, with links to its project page and release notes. This idiosyncrasy in Drupal has been submitted as an issue for Drupal 8, so it may become cleared up at some point in the future. In the meantime, if you find yourself in this situation, it would be best to find a substitute for the unsupported project or, if you have the time and knowledge, offer to add a patch to bring it up to date, or perhaps even take over as the project maintainer.