I have been running into a really strange issue lately when baselining or troubleshooting HTTP applications.
Seeing Page Not Found, or other typical HTTP errors isn't new to me, but when I would reference Wireshark, or any other HTTP troubleshooting tool, there is no reference to the error. OK, thats weird.
The other day, I went to the Linksys/Cisco technical support page and ran into a Page Not Found error, and thought I would capture it with Wireshark to see if I can find the legitimate HTTP 404 error. Guess what, not there.
Perfect. Nows here's my chance to investigate further and try to determine how you can get a Page Not Found error, but Wireshark, TamperData and other tools don't see it.
The short answer is that the webserver has probably been configured to redirect the visitor to its own Page Not Found HTML page.
That means that you will see either an HTTP 200 OK, or HTTP 302 Redirect in your HTTP tool of choice.
Hopefully this little tidbit will help you in your HTTP baselining and troubleshooting.