For the alternative Ending you need the Hacking Tool that you get from the last Sidequest with the Tolga Brothers (it’s at their last Safezone, on a Barrel), with it you can open the Mystery Boxes.

  1. First  open this Box, inside are Nuclear Codes.
  2. Next you need to find the Military activation code here from a corpse lying underwater
  3. and lastly the Military transport Key from a Box near the Behemoth Freak.
  4. then Take all Items to this Truck and see…

You also get the Team Dying Light Outfit for unlocking this more or less unspectacular Ending.