surreptitious

marked by quiet and caution and secrecy; taking pains to avoid being observed

• “a furtive manner”
• “a lurking prowler”
• “a sneak attack”
• “stealthy footsteps”
• “a surreptitious glance at his watch”
• “someone skulking in the shadows”

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”