| notice(noun, verb, mark, consider, see) |
| operative(noun, person, plant, investigate, radical) |
| sight(verb, mark, consider, see, note) |
| agent(noun, person, plant, opposition, investigate) |
| informer(noun, person, information, plant, opposition) |
| peer(verb, consider, sound, see, note) |
| regard(verb, mark, consider, see, note) |
| look(verb, consider, see, note, regard) |
| perceive(verb, mark, consider, see, note) |
| catch sight of(verb, expression, idiom, phrasal verb, consider) |
| eavesdrop(verb, observe, investigate, bug, listen) |
| examine(verb, study, mark, consider, see) |
| infiltrator(noun, person, plant, mole, detective) |
| investigator(noun, person, plant, agent, shadow) |
| case(verb, study, consider, see, regard) |
| double agent(noun, expression, idiom, person, plant) |
| investigate(verb, study, check, consider, sound) |
| look at(verb, expression, phrasal verb, mark, consider) |
| meddle(verb, follow, discover, watch, bug) |
| peek(verb, consider, see, note, regard) |