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

furtive, sneaky, stealthy, sneak

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

concealed

WordNet sense 3 (conducted with or marked by hidden aims or methods):

clandestine, cloak-and-dagger, hush-hush, secret, undercover, underground, hole-and-corner, hugger-mugger

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

covert