If you're missing the key to access the last sim unit room, it's the security key you can find on Engineering. (More information here.)

If it is a simulation chip you're missing look at the maps below. Only red assassins carry sim unit chips. 'A' marks the spot where you can first find the red assassins, and 'B' the place where they run to after seeing you. The maps were created by D'Arcy. (Read more here.)


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