This is a pretty cursory look into some other possiblities of refererless traffic. Given an ordinary browsing session or interaction from an end user, what else could be leading to HTTP requests without the referer header?
There's probably room here to do some investigation as to what Google is doing to make outbound links from mail.google.com completely drop the referer. (Also, what about other web based mail clients? Yahoo? MSN? Aol? Corporate Outlook?)
It looks like the intermediate 301's referer is being passed thru (For example, all links on Twitter get wrapped as a t.co link, and that's what shows up on the server). I'd imagine that analytics being mined are intelligent enough to collapse twitter.com and t.co as the same social origin.
Do you imagine 1, 6, and 7 being something the mass market would be using? Or, do you suppose that it is all of these little cuts that becomes that nearly 70% of traffic being without a header?
11) There used to be an issue in select browsers (not sure how prevalent it is anymore) that when a user opened a link in a new tab/window, that the document.referrer would not be set correctly.
