There is only one Lost Chest per outpost. This means if a cave has more than 1 outpost, there is 1 chest for each.
Lost Chests are often hidden.
To find Lost Chests you may sometimes have to mine your way to where the chest is at as it may be hidden inside something else. You can also use High Yield Dynamite for this or the Goliath Pick, which removes a broader area of ground compared to other picks.