This method also returns false in case you set the value to false, so in order to have a proper fault mechanism in place you need to check the result code to be certain that a key really does not exist in memcached.
<?php
$Memcached = new Memcached();
$Memcached->addServer('localhost', 11211);
$Memcached->set('key', false);
var_dump($Memcached->get('key')); // boolean false
var_dump($Memcached->getResultCode()); // int 0 which is Memcached::RES_SUCCESS
?>
Or just make sure the values are not false :)