FireStats is telling me weird things

Normally when a visitor is referred from another website, that website’s URI is in the “Referrer” field. If there is no referrer, it says “unknown”. However, for the past week or so (after upgrading to 2.1) I’ve had 23 people visiting the page that lists all the plugins I’m using (Viper’s Plugins Used), with no referrer information at all. No URI, no “unknown”.

That’s not all. Those visitors of the page (plugins used) are referred to the search results page, search word: “None”. At first I thought it was a coincidence, but 23 times is no coincidence. The user agent string is always “Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)”. And that’s always where the visit ends. Plugins page, search results, poof…gone.

Does any know what exactly is going on? Is it FireStats, Viper’s Plugins Used, strange bots, or something entirely different? Nothing weird happens when I view the page in the latest versions of Opera, Firefox and IE, logged in or out, so I’m absolutely clueless.

FireStats screenshot here: https://i9.tinypic.com/42i54j8.gif