On this page of our game guide to Batman: Arkham Knight we explain how to rescue the missing ACE Chemicals workers. Batman needs to sneak around and use the detective mode.
Follow the game's suggestion and activate detective mode. This will allow you to notice five enemies near the terminal on the left. For now, ignore the rest of the enemies for now and don't jump down, because the tanks parked there would kill you very quickly.
The easiest way to reach the thugs near the terminal is to use the ventilation shaft. Go down to the tunnel and use it to silently execute two enemies that patrol the area. Next, deal with the single guard who stands the farthest from the terminal. He will not move, so you can easily perform a takedown.
Lastly, use the ventilation shaft again to approach the two thugs standing right next to the terminal. It is best to use Fear Multi-Takedown, which will allow you to eliminate them with zero risk.
After securing the area, use the terminal. Use your grapnel to get on top of the highest building in the factory. After the hero throws a Batarang, start searching a good place for scanning the area. You will know it by the grey bar that appears after pressing the appropriate key/button. Identify five lost workers. Their locations will be marked on your map.