When you go to your blog's Stats page and look in the section "Search Engine Terms", you may see a larger than usual number of "Unknown search terms" listed.
Very recently, Google started to rapidly expand the number of searches that it encrypts, which results in a higher proportion of “Unknown search terms” in your stats. According to some sources, this expansion will eventually result in encryption of all Google searches, so keywords may no longer be accessible.
This is being done for privacy reasons by Google when someone searches at Google.com, before a visitor arrives at your WordPress.com site. Therefore we don’t have any way to unhide the search terms.
We recognize this means a loss of stats information for you and we will look for other ways to show you how users arrived at your site.