Reported on Steam:

Keys on Floor 3 and Core?

In the Hacking Post screen where you can see the map of the whole station and all of your active quests, it says each district on the 3rd Floor and the Core itself have a key to be found. I have have played quite a bit, but haven't been able to find these keys (Edit: It shows "0/1").

I found the three decryption keys that make the Core easier, so I suppose it is possible that those are the keys referred to for the Floor 3 districts and the display is just bugged, but what about the Core? Or are they just super rare?