Move without sound, hide in shadow or crowd, or make yourself invisible to people actively looking.
The hard part is not finding cover. The hard part is staying in it — breathing slowly, not moving when the torchlight swings close, not tensing when the guard stops two feet away and listens. The body wants to run. The body is wrong. Stillness is a skill. Crowds are cover. Darkness is not your enemy if you know how to become part of it.
Sneak is the skill of controlling your presence — moving silently, hiding your body, blending into a crowd, trailing someone without closing distance. Make a Sneak (Agility) Trial when your position or movement could give you away. For manipulating objects without being detected, see Cunning (Agility).
Examples: crossing a guarded courtyard in darkness; holding still in a cupboard while a search passes through; trailing a mark through a market without closing the distance; slipping out of a room before the lantern turns.