Yahoo has switched to secure search by default, meaning that the search traffic your analytics program is painting for you is even less complete than it already was.
As you may recall, Google made this move a couple years ago for signed in users, and expanded it to even those who aren't signed in last year. While good for consumers in that it makes their searching habits more private, it's not so good for marketers, as the bulk of their Google referrals started being displayed with keywords "not provided". It is no longer clear what search terms your visitors are using to find your pages, at least through Analytics. Google has continued to provide this information to advertisers and through Webmaster Tools.
According to Danny Sullivan at MarketingLand, Yahoo's version, doesn't only hide the actual keywords searchers are using, but it disguises the referrals you get from Yahoo searches as "direct" traffic. In other words, you don't even know that you're getting Yahoo traffic.
CEO Marissa Mayer announced in November that Yahoo would encrypt all information that moves between its data centers by the end of Q1. It turned on SSL/HTTPS encryption for Yahoo Mail earlier this month.
Bing has also been messing around with secure search, though not to the extent of Google or Yahoo. Last week, news came out that Bing has made the option available, but has not enabled it as the default.
Yahoo's default secure search is apparently still in the process of rolling out.
Image via Yahoo