You can hide your custom Drupal modules from being scanned by the Update Status module by removing your module from the $projects array in hook_projects_alter(). Doing so will prevent "No available releases found" warnings when checking for available updates.

/**
 * Remove our custom modules from the $projects list so
 * the update manager won't check them for needed updates.
 * @param $projects
 */
function MYMODULE_update_projects_alter(&$projects){
  unset($projects['mymodule_machine_name']);
}

References

1. function hook_update_projects_alter (api.drupal.org)