WordNet sense 1 (marked by quiet and caution and secrecy; taking pains to avoid being observed):

sneaky, stealthy, surreptitious, sneak

WordNet sense 2 (hidden on any grounds for any motive):

concealed

WordNet sense 3 (secret and sly or sordid):

backstairs, backstair

WordNet sense 4 (secret or hidden; not openly practiced or engaged in or shown or avowed):

covert