You will unlock this ability, and therefore, the achievement, on mission 4: "The Tower".
After the first act, you will enter to the tower with a gravity lift, once you are in, you will find a distress signal up ahead with pieces of a Spartan armor, interacting with it will trigger a cutscene, once it is done, you will unlock the Threat Sensor, which will come in very, very handy for this mission.