noun (n)

• an investigator who is employed to find missing persons or missing goods(noun.person)

  source: wordnet30
• an instrument used to make tracings(noun.artifact)

  source: wordnet30
• (radiology) any radioactive isotope introduced into the body to study metabolism or other biological processes(noun.artifact)

  source: wordnet30
• ammunition whose flight can be observed by a trail of smoke(noun.artifact)

  Synonym:
  tracer bullet
  

  source: wordnet30
• One who, or that which, traces.(noun)

  source: webster1913
• A person engaged (esp. in the express or railway service) in tracing, or searching out, missing articles, as packages or freight cars.(noun)

  source: webster1913