What3Words is a terrific app, and the data is some of the most accurate out there. While I don’t have knowledge of missing, I do know that it has been credited with several successful saves of persons who have been lost and are not on main roads. (It’s also helpful to locate a person who calls 911, but is not inside the residence – think about like a farmer in a field – 911 will send them to the house, but What3Words will direct them to the field.)
There’s a feature in the app where you can send your 3 words to another user and it will give directions. As a dispatcher, I have used to it help callers find their location when they have an accident and don’t know where they are. What3Words interfaces with the RapidSOS portal, which is another tool to help determine a callers location.