Based on materials from techradar.com
If you are smart enough, you can dig through your application files to find something related to future functionality – something that the developers have already laid the foundations for, but have not yet fully revealed to the world.
In Android Police went through version 4.8 of the Google app (which contains Google Now, OK Google search, and more) and found a code that indicates the imminent appearance of the offline version.
This means that you can perform certain actions in the absence of Wi-Fi and data transmission. Obviously, it doesn't look like web search is included here, unless Google is going to store the entire Internet in your phone's cache.
Android Police say that offline 'OK Google' will be able to make a call, send text, play music and turn on Wi-Fi on demand. Perhaps Google will add more options in the future.
Apparently, the code assumes that voice-only volume and brightness control will be available soon. So you can shout at the phone and make it go silent or go out.
As Android Police say, this should not be considered a fait accompli. Google can simply test features without planning to launch them soon. But while Siri and Cortana are constantly improving, OK Google and Google Now can't stand aside.