The Zero Essential is usually forgotten by those who mindlessly regurgitate the ten essentials, perhaps because you don’t buy it in a store, and you don’t even take it with you. (For more on this issue see The Ten Essentials Farce.)
The Zero Essential is something you leave at home. It is the information about your planned activity and planned return time left with a trusted friend or family member. It allows Search and Rescue teams to begin a SAR effort at the earliest possible moment. The difference between a rescue and a recovery of a person from the wilderness is often a race against the clock.
The Zero Essential works unlike anything you might have in your backpack because it can’t be lost by you, or be broken, or run out of power, or be inoperable because of your injury or illness.
Don’t leave home without leaving it home.