undercover, cloak-and-dagger, hole-and-corner, backstair, hugger-mugger, clandestine, surreptitious, concealment, under-the-table, on the quiet, more...

overt, visible, open, apparent, obvious

spy, undercover agent, operative, informant, covert operative, secret agent, espionage agent, deep cover agent, more...

nice and secret,   thinly veiled,   secret or hidden,   hidden from view,   barely hidden,   less visible,   without being seen,   hidden from,   not openly,   being secretive