Stealth Navigation is Syndicate's Sneak Mode. It makes you harder to spot and lets you aim, shoot and throw from cover. You have it from Sequence 1.
Stealth Navigation sits at position 3 of 19 in the Stealth tree. Stealth Navigation costs no skill points. The twins learn it during Sequence 1. Jacob and Evie each have their own skill tree, so you buy Stealth Navigation once for each twin.
Air Assassination
Drop onto a target from above and kill him in a high-profile strike.
Assassin Steps
Greatly reduces fall damage, silences your landings and makes your movement much quieter.
Chameleon
In Sneak Mode, Evie becomes invisible while standing still; only enemies very close by can spot her.
Double Assassination
Kill two nearby unaware enemies at the same time.