When you receive a foretaster e-mail with customer's flow which contains Google search, you usually see the search term (keyword) just underneath the circle. But sometimes there is no keyword ... why?
When people search using https://www.google.com (notice the s after http, meaning secure => https) and/or they are logged into their Google accounts, Google does not provide data about the search terms (keywords). At the moment, this is the case for about 20-30% of all searches.
You won't be able to see these keywords even in Google's own Google Analytics.
Read more about this issue at the Google Official Blog.