* He moved through the shadows like a phantom, unseen and unheard. (Compares the person to a ghost)
* Her smile was a wolf in sheep's clothing, hiding a cunning plan. (Compares the smile to a dangerous predator)
* He was a serpent in the grass, waiting for the perfect moment to strike. (Compares the person to a snake)
* She was a shadow, slipping in and out of rooms unnoticed. (Compares the person to a shadow)
* His words were like honey, sweet and deceptive. (Compares words to honey)
The best metaphor will depend on the specific context and the kind of "sneakiness" you want to convey.