What is another word for "spy"?

Roget's Thesaurus (1911) lists these synonyms for "spy": ambassador, apparitor, bag, bearer, behold, beholder, bellman, boy, bystander, cable, carrier, chore, commissionaire, correspondent, courier, crier, dak, delivery, diplomatist, electronic, emissary, envoy, errand, estafette, express, eyewitness, flag, gawk, gentleman, herald, information, informer, internuncio, legate, letter, look, mail, marshal, messenger, news.