covert -
a covering that serves to conceal or shelter something; "a screen of trees afforded privacy"; "under cover of darkness"; "the brush provided a covert for game"; "the simplest concealment is to match perfectly the color of the background"
unseeable,
invisible impossible or nearly impossible to see; imperceptible by the eye; "the invisible man"; "invisible rays"; "an invisible hinge"; "invisible mending"
concealed hidden on any grounds for any motive; "a concealed weapon"; "a concealed compartment in his briefcase"
backstair,
backstairs,
furtive secret and sly or sordid; "backstairs gossip"; "his low backstairs cunning"- A.L.Guerard; "backstairs intimacies"; "furtive behavior"
black marked by anger or resentment or hostility; "black looks"; "black words"
secret the next to highest level of official classification for documents
collusive,
conniving acting together in secret toward a fraudulent or illegal end
disguised,
cloaked,
masked covered with or as if with clothes or a wrap or cloak; "leaf-clothed trees"; "fog-cloaked meadows"; "a beam draped with cobwebs"; "cloud-wrapped peaks"
secret the next to highest level of official classification for documents
subterranean,
subterraneous,
ulterior lying beyond what is openly revealed or avowed (especially being kept in the background or deliberately concealed); "subterranean motives for murder"; "looked too closely for an ulterior purpose in all knowledge"- Bertrand Russell
under wraps kept secret; "the plan was kept carefully under wraps"
covert -
(of a wife) being under the protection of her husband; "a woman covert"
