Use the adjective stealthy to describe something or someone that is marked by secrecy, even sneakiness. Ninjas are stealthy, moving quietly and cautiously, which makes them excellent spies.
When something is done in a stealthy way, the motivation is almost always to escape from being noticed. If your cat is walking so lightly that you can’t hear her footsteps, she's not necessarily being stealthy on purpose, she’s just a graceful cat. However, if you see your cat creeping forward toward a mouse, trying not to scare it away before she can pounce, then she's definitely being stealthy.