Did you ever get a referral from Google with it being blank? This is the first time I have seen a thread on this topic over at WebmasterWorld. Several people chime in that the answer is that the searcher used the "I am feeling lucky" button to be referred to that page. Even GoogleGuy:
"I would guess an I'm Feeling Lucky search as well. I suppose you could always try it out by constructing a search that would return you at #1, and then checking your server log. Let us know what you find out. :)"
Ukgimp's tests proved this theory to be fact.
