Drupal - What is the template suggestion for node 'teaser' view mode?

I don't think there is one by default but you can easily add one in your template.php file:

function MYTHEME_preprocess_node(&$vars) {
  if($vars['view_mode'] == 'teaser') {
    $vars['theme_hook_suggestions'][] = 'node__' . $vars['node']->type . '__teaser';   
    $vars['theme_hook_suggestions'][] = 'node__' . $vars['node']->nid . '__teaser';
  }
}

That will let you use a template file like: node--[type|nodeid]--teaser.tpl.php