IDIOMS

smoke signal

(idiomatic, often pluralized) An indirect message or indication, especially concerning a future event.

Example: 1971, A. W. Lintott, "Lucan and the History of the Civil War," The Classical Quarterly, vol. 21, no. 2, p. 492:
  However, there are still signs of the careful study of sources—the smoke-signal which brought Caesar news of the Pompeian escape (279; Caes.3.65.2) and the second fortification inside the camp which Caesar attacked (288; Caes.3.66.4-5).
1950, "The Wooing of Mao," Time, 11 Sep.:
  By every kind of wigwag and smoke signal in the language of diplomacy, the Administration seemed to be trying last week to tell Chinese Communist Boss Mao Tse-tung that he had nothing to worry about from the U.S.
A type of flare or combustion device sometimes used as a distress signal.
1965, "Operation Rescue," Time, 12 Mar.:
  A Huskie helicopter was whirring over the Gulf of Tonkin in search of Cullen when Vietnamese 1st Lieut. Nguyen Van Phu, who ditched his flaming Skyraider near the spot where the U.S. pilot went down, fired a smoke signal to attract its attention.