smoke signal

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun often pluralized A method of long-distance communication sometimes used in ancient and undeveloped societies, consisting of messages conveyed by means of columns or intermittent puffs of smoke.
• noun idiomatic, often pluralized An indirect message or indication, especially concerning a future event.
• noun A type of flare or combustion device sometimes used as a distress signal.