A wrong behaviour was fixed, when there was no module route and no route matched the given request. Previously, the router returned an unmodified request object, so the front controller just displayed the default controller and action. Since Zend Framework 1.10, the router will correctly as noted in the router interface, throw an exception if no route matches. The error plugin will then catch that exception and forward to the error controller. You can then test for that specific error with the constant Zend_Controller_Plugin_ErrorHandler::EXCEPTION_NO_ROUTE:

/**
 * Before 1.10
 */
    public function errorAction()
    {
        $errors = $this->_getParam('error_handler');

        switch ($errors->type) {
            case Zend_Controller_Plugin_ErrorHandler::EXCEPTION_NO_CONTROLLER:
            case Zend_Controller_Plugin_ErrorHandler::EXCEPTION_NO_ACTION:
    // ...

/**
 * With 1.10
 */
    public function errorAction()
    {
        $errors = $this->_getParam('error_handler');

        switch ($errors->type) {
            case Zend_Controller_Plugin_ErrorHandler::EXCEPTION_NO_ROUTE:
            case Zend_Controller_Plugin_ErrorHandler::EXCEPTION_NO_CONTROLLER:
            case Zend_Controller_Plugin_ErrorHandler::EXCEPTION_NO_ACTION:
    // ...