In the name of data privacy Google runs all web search through SSL. The result? Well; one of the results is that you'll no longer see keywords for natural search in your analytics.
Sometimes you do see keywords in your analytics, though. So what's going on? How come you see some keywords but not others.
There are a few answers. The first is that right now (this may change) is that keywords are still being passed from Google Image search. Needless to say, it might be another search engine other than Google responsible for the traffic too.
However, it's also possible that a network owner has stripped the SSL off. Starbucks do this. In response, Google shows a warning to searchers - your data might not be secure. In this scenario Google even refuses to show personalised results. That would matter to you if you let Google prompt you on emails, calendar invites and Google+ messages.