Well, if the person has the cookies on their computer, than information will be passed when you call $_COOKIE['user'] and $_COOKIE['pass'].
So, you can always test to see if those variables are not empty, and if they are redirect them to your signin page with the invalid marker.
PHP Code:
$username = $_COOKIE['user'];
$password = $_COOKIE['pass'];
if(empty($username) || empty($password)){
header('Location: ../signin.php?error=invalid');
}
I'm not sure if this is what you were looking for. If it's not, please correct me.