Emphasizing quiet and secrecy:
* A shadow slipping through the night.
* A whisper in the wind.
* A ghost in the machine.
* A ripple on a still pond.
* A feather falling on a silent world.
Emphasizing skill and deception:
* A chameleon blending into its surroundings.
* A magician's sleight of hand.
* A master spy playing a game of chess.
* A wolf in sheep's clothing.
* A smoke screen obscuring reality.
Emphasizing suddenness and surprise:
* A lightning strike from the blue.
* A bomb detonating out of nowhere.
* A predator springing from the shadows.
* A sniper's bullet finding its mark.
* A thief snatching something in the blink of an eye.
Emphasizing subtlety and finesse:
* A needle threading through a haystack.
* A key unlocking a hidden door.
* A secret code deciphered by a skilled mind.
* A tapestry woven with intricate details.
* A dance of shadows and light.
The best metaphor for your needs will depend on the context and the specific meaning you want to convey.