This achievement is only obtainable in mission three, Day 31 - Lost.
In the beginning of this level your dispatch will instruct you to scan the two locations on the map with your Terra-Scanner to locate the lost cave. First, go to the location on the left (Area 1) and use the Terra-Scanner while standing directly on the blue dot on the map. When using the Terra-Scanner, make sure you hold left click until the animation is complete.
Once you have scanned Area 1, go back up and to the right and repeat for Area 2.
Keep in mind that achievements may take a moment to pop in this game.