Mysterious Boxes are special locked crates scattered throughout the map of The Following. They are roughly the size of a large toolbox and are painted in a dark orange coloration. These boxes contain a variety of unique developer blueprints.
They will remain locked until the player acquires the Hacking Device from Tolga and Fatin after the quest Secret Project. Attempting to activate them prior to that will simply result in the Mysterious Box being marked on the player's map.
There are currently six known locations for mysterious boxes, as it is unclear if there are others that have yet to be found by players.
With the tracking device that Tolga and Fatin give you at the beginning of their questline the player can track the location of each mysterious box based on their proximity to it.
Czesiu's Super Zombie GrenadeImmediately after acquiring the hacking tool upon completion of the Secret Project quest the mysterious box can be found by a small vehicle not far from the tunnel entrance. This grenade causes walkers and infected to float and fall after a period of time.
Located at the top of a water tower in the northwest corner of the map, look for a parachute draped over a metal strut.
Through the graveyard above a volatile nest in the east center of the map. The box is located on the easternmost island.
Timmy's Ballet Grenade
Located on an island in the middle of a lake towards the centre of the southern part of the map. East of the racetrack and west of factory buildings.
Located off the eastern coast of the lighthouse, submerged in shallow water several meters away from the shore. A parachute can also be seen on the beach marking its location.