Just did a check on about 50 old pages and not receiving a 404 page not found on any, which could be why we still have 1700 pages indexed on Google but only about 300 live pages on our site...
Our old Product Pages are just a blank page, and if you go into the page source code, it is blank besides the number "1" on it. :
But if you go to a faulty/old URL from another site it states page not found, and the page source code is as below:
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<title>404 Not Found</title>
<p>The requested URL /h was not found on this server.</p>
Even Web Assist has a 404 page http://www.webassist.com/error_404.php