The new version of the app does not allow you to change the smart home monitor “security mode” based on anything except time of day or manually toggling it in the app. You cannot change it based on presence.
You could in the classic app. But you can’t in the new one. It now matches the way security mode works for the ADT model hub, but they have applied that to all of the hub models.
You can write to support and tell them that you would like to see it changed, quite a few people have. But for now it’s just not an option.
