IDK about the rest, but this is the one I found by accident and it was with the phones, no computer.
- Go to the building south of District Union Arena stronghold.
- Go to the center of the complex and face south.
- Go inside the building and interact with a computer.
- Turn around and head back north, go across the clearing, and into the building.
- Interact with another computer.
- Hunter will spawn in the clearing. Kill it for Crimson Mask.