General synonyms:
* Anonymous
* Unidentified
* Unknown
* Disguised
* Concealed
* Hidden
* Secret
* Undercover
More specific synonyms:
* Cloaked (implies a physical disguise)
* Masked (implies a physical disguise covering the face)
* Unrecognized
* Unrevealed
* Unidentified
* Under an assumed name
* In disguise
The best synonym to use will depend on the context of your sentence.