When a product is marked as "hidden", or has a product status with the "hide" option set, it should present a 404 page on the front end for search engines.

Currently if you visit the product page via it's custom URL you can still see it, this allows it to be indexed in Google and customers can find the page even when it should be hidden.

If a store user is logged into the admin in the same browser, then show the product page as if it would appear to the customer