Takedown Tagger is a gear perk that tags nearby enemies after a takedown kill. It keeps you aware of who is around after each silent kill, so you can plan the next one.
After a takedown kill, enemies near you get marked. You see who might have noticed and who to take out next.
Wear the Cyber-Eye or Decontaminant Mask in the head slot. Gear perks are fixed to the piece of clothing that carries them, so you get this one by wearing that piece, not by unlocking it in a menu.
Quiet Confidence reduces your noise after takedowns, and Righteous Path refills your magazine if things go loud.