tedster - 7:42 am on Nov 23, 2011 (gmt 0)
Lucy, all anyone needs to do is be logged in to any Google account. Then Google Search switches to SSL (https protocol) and Google does not include the query terms in the referrer string. You're right - I'm sure all those people were not finding a private browsing method on their own.
This is the second report I've heard of receiving over 30% of referrers with keywords not specified. Of the sites I'm working with, the highest has been 11%.
I am frustrated that some hidden gems that the page is ranking for may not be available to me now. But other than that frustration I'm just trusting that, at volume, the distribution of regular query terms is going to stay approximately proportional to the logged out user.