Unused/Unchecked XML site map urls not returning 404

it appears in my case for whatever reason that line 182 in class-sitemaps.php is not working correctly and setting a 404 status code for invalid or empty site maps.

$GLOBALS[‘wp_query’]->is_404 = true;

However if this line is replaced with the code below the correct header is returned for non-existing site map files

global $wp_query;
$wp_query->set_404();
status_header(404);

Problem solved but do not fully understand why $GLOBALS[‘wp_query’]->is_404 = true; is not working correctly – > perhaps W3 Total Cache?

The reason why this could be important is if a spammer knows you are using YOAST, he spam links to “non-existent” pages which appear to search engines as normal pages with 200 O.K. status.