During Mission 3: Embedded
, at the start of the mission you will need to pick up a cinder block as part of a disguise to get past the guards. You must keep this same cinder block with you throughout your entire journey through this mission.
During certain parts, you will need to leave it behind including when you are shooting any enemies, and running certain objectives like planting and detonating C4. Climb down the ladder at the end of the level holding the cinder block as you interact to unlock this.
Possible on any difficulty, but recruit is probably recommended to make it as easy as possible.