Scenario: you’ve upgraded Drupal core to the latest version (via composer) but Drupal thinks an earlier/previous version is the most recent, despite running drush updb, clearing the cache, running cron etc.
e.g. you’ve upgraded Drupal core to 8.3.1, but still see a “version 8.3.0 available” warning on /admin/reports/status
Simple fix: go to /admin/reports/updates and click “Check Manually” and you’ll see a progress bar and it will pull in the latest list of packages and the warning will go away.