Takedown Surveillance is a gear perk that tags nearby mechs and security cameras when you make a takedown kill. It helps you spot automated threats as you work through an outpost quietly.
Each takedown kill marks nearby mechs and cameras. You learn what is watching the area right after you clear a guard.
Wear the Extraction Gear or MK VII Tac Comm 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.
Takedown Tagger tags nearby enemies after takedowns, so between them a single takedown shows you most of the threats.