Implement hook_field_extra_fields() for information on user page

By using hook_user_view to inject content into an user page without declaring an extra field you are totally ruining efforts of site builders who want to use fields and view modes to show their users on their websites.

Here is a snippet to help :

function persistent_login_field_extra_fields() {
  $extra = array();
  $extra['user']['user']['display'] = array(
    'persistent_login_warning' => array(
      'label' => t('Persistent Login warning'),
    ),
  );
  return $extra;
}

function persistent_login_user_view($account, $view_mode) {
  global $user;
  if ($user->uid == $account->uid || user_access('administer persistent login')) {
    $settings = field_extra_fields_get_display('user', 'user', $view_mode);
    if (!empty($settings['persistent_login_warning']['visible'])) {
      // Do you stuff here
    }
  }
}

I hope you will find this useful even if I didn't have the time to make a proper patch.