Just doing a bit of grinding in Judiciary Square this morning and came across a random Hunter?
Raging that I didn't have any special ammo for my Crossbow...
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Just went to the same place and after collecting on the nearby roof and coming back down to the alley I got him to appear. I killed him but no ivory key, just high-end chest piece.
Hunters can spawn in a lot of places during the evening / night, they have been spotted in a lot of different places and have nothing to do with the Ivory keys.
They can however drop the masks at random if you manage to oneshot them but this doesn't seem to have a 100% chance of happening.