Chris – referrers aren’t shown when there isn’t one. Lately I’ve noticed an increase in spam comments with a referrer and noticed a pattern in my logs from the IPs of those commenters.
The spammers have a new tool, it’s probably desktop based and probably launches a browser with the post in question. Those comments don’t have referrers.

I didn’t have a specific page ready for the plugin when I created it, so I do need to point it at the right one now. Doesn’t have to be the wporg plugin page though. Thanks for noticing!