This is a story related achievement and is unmissable.
This is located when you use the batarang scanner to locate Scarecrow, and takes place just after rescuing the police officer from the thugs on top of Falcones Yard , quite close to the start of the main campaign.
After this, head outside and a cutscene will start.
You then need to solve a small 'puzzle' to find scarecrows location and pop the achievement.
*** Spoiler - click to reveal ***Not sure if it's possible to not understand it, but for those who may want help he is located at ACE chemical plant on the eastern side of the map.