surreptitious
adj.
marked by quiet and caution and secrecy; taking pains to avoid being observed
a furtive manner|a sneak attack|stealthy footsteps|a surreptitious glance at his watch
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