Symfony redirect to external URL
Answer to your question is in official Symfony book.
http://symfony.com/doc/current/book/controller.html#redirecting
public function indexAction(){ return $this->redirect('http://stackoverflow.com'); // return $this->redirect('http://stackoverflow.com', 301); - for changing HTTP status code from 302 Found to 301 Moved Permanently}
What is the "URL"
? Do you have really defined route for this pattern? If not, then not found error is absolutelly correct. If you want to redirect to external site, always use absolute URL format.