A detective who wears civilian clothes on duty.

Roget 668: warning, early warning, caution, caveat; notice etc. (information) 527; premonition, premonishment^†; prediction etc. 511; contraindication, lesson, dehortation^†; admonition, monition; alarm etc. 669.    handwriting on the wall, mene mene tekel upharsin, red flag, yellow flag; fog-signal, foghorn; siren; monitor, warning voice, Cassandra^†, signs of the times, Mother Cary's chickens^†, stormy petrel, bird of ill omen, gathering clouds, clouds in the horizon, death watch.    watchtower, beacon, signal post; lighthouse etc. (indication of locality) 550.    sentinel, sentry, , watch, watchman; watch and ward; watchdog, bandog^†, housedog^†; patrol, patrolman, vedette^†, picket, bivouac, scout, spy, spial^†; undercover agent, mole, plainclothesman; advanced guard, rear guard; lookout.    cautiousness etc. 864.    monitor, guard camera, radar, AWACS, spy satellite, spy-in-the-sky, U2 plane, spy plane.    ... show more

Polish: tajniak