smoke signal 
noun
1A column of smoke used as a way of conveying a message to a distant person.Many of their traditions and their original methods of communication - their language, smoke signals, and messages carried by people on foot - have survived....
- Tribes in Kurdistan set up a primitive warning system with observers and smoke signals to warn the most likely targets of the approach of British aircraft.
- The smoke signals from the Vatican were certainly this week's most newsworthy communicative acts.
1.1An indication of someone’s intentions or views: a series of political smoke signals has aroused hopes for a fresh initiative on Northern Ireland...
- Various smoke signals have been sent up, but in truth, nobody knows. This is the type of thing that Bertie always, always keeps in pectore until the very last moment.
- As ever in Northern Ireland, however, the smoke signals are confused and indistinct.
- If you read The Sunday Times last week, there were smoke signals that gave you a pretty good idea what that might be.