Synonyms: sleuth, sleuthhound
Definition: a detective who follows a trail
Similar words: detective
Definition: an investigator engaged or employed in obtaining information not easily available to the public
Synonyms: spy, stag, sleuth, snoop
Definition: watch, observe, or inquire secretly
Similar words: monitor, supervise
Definition: keep tabs on; keep an eye on; keep under surveillance
Usage: we are monitoring the air quality; the police monitor the suspect's moves