As 9to5Google notes, stories featured on the Discover tab inside the Google Android app currently include related hashtags for some. A story about eye-tracking functionality on OLED TVs, for example, includes #OLEDTV underneath the headline and article blurb. An iPhone story includes #iPhone, #Apple, and #iOS. Clicking on any of the hashtags takes you to more news stories about that topic.
Discover is not an option on the Google iOS app, and the hashtags did not appear on Google.com via Safari or Chrome for iOS.
9to5Google notes that it's unclear whether this is an actual test for an upcoming feature or just a glitch. But Google already quietly added hashtags to its YouTube titles on both the web and mobile browsers.