there's probably sentient life, but maybe just a forest fire.
In any case, in a Western Marches-esque game, smoke might be a Big Deal.
In the first case, the PCs see a column of smoke in the distance. If there are several such columns and they're in the right direction, they could be coming from $HOMETOWN. In this case, they're a good way to find your way home when lost in the woods (if the weather conditions are cooperative).
If there are fewer columns, or they're in places the PCs might otherwise not expect, this is probably a good way to locate native encampments, for whatever purpose one might choose (avoidance, trade, raiding, &c).
And a single plume of smoke is likely a campfire with attendant small party of sentients who might be met on even terms...
And now we come to the second case: what's good for the PCs is good for the monsters. A fire might keep the cold and the wolves at bay, but bring the goblins a' calling... The campfire becomes a resource/risk management decision, especially if you (say) reduce natural healing rates when sleeping in the cold.