Removed sitemap creates what looks like 404 page, but doesn't send a 404 header

If you exclude a post type form being included in the sitemap, the old URL of that post type sitemap looks like a 404 page, but sends a 200 status, not a 404 status:

e.g:

http://www.qualtechelectrical.com.au/slide-sitemap.xml

I’ve seen this happen on about 5 different sites now, some with plugin, some with none, on various different version of wordpress

This has caused major issues on some sites where the 404 looking page, is actually a duplicate of the home page, thus a duplicate homepage gets indexed in Google.

Thanks for you help

http://wordpress.org/extend/plugins/wordpress-seo/