Add a subnav to a Drupal theme from the secondary menu and include a "back" button if the active trail is in the sub nav.

drupalSubNav.php

<?php
function TEMPLATE_preprocess_page(&$variables, $hook) {
  $variables['secondary_menu'] = menu_secondary_menu();
  
  $activesub = false;
  foreach($variables['secondary_menu'] as $item) {
		if(isset($item['attributes']['class']))
			foreach($item['attributes']['class'] as $class)
		  	if($class == "active-trail")
		  		$activesub = true;
	}
	
	$active_trail = menu_get_active_trail();
	if(sizeof($active_trail) > 2) {
		end($active_trail);
		$parent = prev($active_trail);
		
		array_unshift($variables['secondary_menu'], array('title' => '<<Back ', 'href' => $parent['href']));
	}
}
?>