The Division 2 Faction Keys Guide.
Factions keys are used to open special high-end faction lockers that drop level-appropriate loot. The best way to farm those keys is by heading into tunnels and underground locations such as sewers to look for special lockers as displayed in the screenshots. One of such location yielded 4 different faction keys. They do drop as a random reward from killing NPC's however the yield is very small. And drop from NPC's are always faction specific.