Synonym of SURREPTITIOUS

Mysterious : difficult or impossible to understand, explain, or identify.
Abstract : existing in thought or as an idea but not having a physical or concrete existence.
Open : allowing access, passage, or a view through an empty space; not closed or blocked.
Secret : not known or seen or not meant to be known or seen by others.
Surreptitious : kept secret, especially because it would not be approved of.

Synonym of Surreptitious is Secret