This is a bonus cache you can find for completing the Fort Southwest Point Adventure Lab. There are five stops on the Adventure Lab and completing each stage you will recieve part of the missing coordinates for this cache. They will be kept in your journal so you can refer back to them anytime after completion.
Just plug in the missing numbers how you were told to in your journal to N 35 51.ABC W 084 31.DEF
You will be looking for a plastic ammo can hidden nearby. Please use stealth as this can be a busy place sometimes and PLEASE hide the cache back better than you found it!