General synonyms:
* Unseen
* Hidden
* Concealed
* Imperceptible
* Ethereal (suggests something almost otherworldly)
* Phantom (suggests something ghostly or unreal)
More specific synonyms:
* Transparent (for things that allow light to pass through)
* Inconspicuous (for things that are not easily noticed)
* Indistinguishable (for things that blend in with their surroundings)
* Undetectable (for things that cannot be detected by any means)
* Elusive (for things that are difficult to find or see)
The best synonym to use will depend on the specific context and the meaning you want to convey.