Tesla announced in a blog post that Sentry Mode must be enabled from the safety controls menu each time a driver wants to use it. The feature started to be rolled out yesterday to U.S.-market Model 3 vehicles, followed by Model S and Model X cars built after August 2017. These have the Autopilot 2+ system required for Sentry Mode to work.
As for Dog Mode, it's not a guard dog feature that will have the car barking — it's a cabin temperature control mode for the occasions when you have to leave your dog in the car. It will cool the cabin so that pets can stay in the car in comfort. In addition, the mode also displays a notification on the big screen, telling onlookers that the pets inside the car are cooled and there's no need to break a window to help them. You just have to hope a Good Samaritan would think to look at the screen before grabbing a hammer.
Introducing Dog Mode: set a cabin temperature to keep your dog comfortable while letting passersby know they don't need to worry pic.twitter.com/xFU6MGZT53— Tesla (@Tesla) February 14, 2019