General Synonyms:
* Secret
* Covert
* Clandestine
* Surreptitious
* Underhanded
* Furtive
* Sneaky
* Hidden
* Unseen
* Invisible
More Specific Synonyms:
* Elusive: Difficult to find or capture.
* Shadowy: Mysterious and secretive.
* Discreet: Prudent and careful in avoiding offense or notice.
* Sly: Clever and cunning in a devious way.
* Devious: Using dishonest or underhanded methods to achieve something.
Figurative Synonyms:
* Ghostly: Like a ghost, unseen and unnoticed.
* Shadow: Something that moves unnoticed, like a shadow.
* Catlike: Moving with the stealth and agility of a cat.
The best synonym to use will depend on the specific context of your writing.