When a product is deleted or downgraded to a status of zero the catalog displays a "Product not found" text. However, the HTTP status code for that product URL is still 200. If a product page truly no longer exists, a HTTP status code of 404 is desired to indicate to search engines that the page no longer exists and the page should be deindexed accordingly.
Sniff the headers of an active product, or use the 'Fetch as Google' feature found in Google Webmasters account for the website being tested. The HTTP status code will be 200. Delete the product or reduce the product's status to zero and perform the same tests. The HTTP status code will remain 200 or OK.