He is using Wordpress so I assume PHP: $_SERVER['HTTP_REFERER'];

http://php.net/manual/en/reserved.variables.server.php

HTTP_REFERER "The address of the page (if any) which referred the user agent to the current page. This is set by the user agent. Not all user agents will set this, and some provide the ability to modify HTTP_REFERER as a feature. In short, it cannot really be trusted."

It's WP Greet Box and it's fairly controversial here [1]. I may remove it. I'm not entirely sure I see the problem with reminding HN users to vote for a piece of content if they found it interesting, but it's not a big deal so if people are going to boycott me over it, I'll dump it.

1. See discussion here http://news.ycombinator.com/item?id=1738183

I liked it, and prompted me to finally get around to registering, so the win is clear for HN as well.