undercover
adj.
conducted with or marked by hidden aims or methods
clandestine intelligence operations|cloak-and-dagger activities behind enemy lines|hole-and-corner intrigue|secret missions|a secret agent|secret sales of arms|surreptitious mobilization of troops|an undercover investigation|underground resistance
Synonym: clandestine, cloak-and-dagger, hole-and-corner, hugger-mugger, hush-hush, secret, surreptitious, underground