Software and strings used in blocking the referring URL
There are many "personal privacy" or "personal firewall" products which use a client-side proxy to modify what a user's browser sends and receives.
AXS was created in 1997 before such client-side proxies were common; it doesn't handle the missing Referer field very well.
Common replacement strings are:
referrer blocked by AdSubtract
Referrer is being replaced by AdSubstract Pro, a program whose main purpose is to strip banner ads. See adsubtract.com.
Referrer is being replaced by Outpost, a personal firewall program. From their FAQ http://www.agnitum.com/support/outpostfaq.html
Why do I see 'Field blocked by Outpost' in my web server logs?
If you see 'Field blocked by Outpost' in your site statistics, it means those visitors have Outpost Firewall installed on their own computers. They have set Outpost to block 'referrers' to protect their privacy and not disclose the site they visited prior to yours. Please note that these users still will be able to browse your website. You can ask your visitors to enable Referrers in their Active Content plug-in to see how they arrived at your site.
This is reportedly done by Zone Alarm Pro; some have reported that Symantec's Norton Internet Security uses this string as well.
Because of the way this is implemented, the referring URL is often logged by AXS as the URL folder following by that string, i.e. "http://www.xav.com/perl/Hidden-Referrer".
"Software and strings used in blocking the referring URL" http://www.xav.com/scripts/axs/help/1012.html