| operative(noun, plant, agent, mole, infiltrator) |
| patrol(noun, plant, agent, operative, informer) |
| picket(noun, plant, agent, operative, informer) |
| plant(noun, agent, operative, informer, mole) |
| scout(noun, plant, agent, operative, informer) |
| secret agent(noun, expression, plant, agent, operative) |
| shadowing |
| sleeper(noun, plant, agent, spy, operative) |
| sleuth(noun, plant, agent, operative, informer) |
| snoop(noun, plant, agent, operative, informer) |
| spook(noun, plant, agent, operative, informer) |
| spotter(noun, plant, agent, spy, operative) |
| spy(noun, person, plant, mole, sleeper) |
| spy agency |
| stakeout |
| tailing |
| trailing |
| undercover agent(noun, expression, plant, agent, operative) |
| watcher(noun, plant, agent, operative, informer) |
| wiretap |