General:
* Enigmatic
* Mysterious
* Abstruse
* Obscure
* Inscrutable
* Vague
* Ambiguous
* Opaque
* Secret
* Hidden
* Unclear
More specific:
* Coded: For messages that are intentionally hidden.
* Allusive: For something that hints at something else without explicitly stating it.
* Symbolic: For something that represents something else.
* Arcane: For something that is known only to a few people.
* Esoteric: For something that is difficult to understand because it is specialized or obscure.
The best synonym to use will depend on the specific meaning you want to convey.