A minor event or action that predicts or foreshadows a future event. His negative remark about marriage was a straw in the wind that suggested he was headed for a divorce.Bill didn't get the promotion and, looking back, I think his very public argument with the boss was a straw in the wind.
A slight hint of the future, as in The public unrest is a straw in the wind indicating future problems for the regime. This expression alludes to a straw showing in what direction the wind blows, an observation also behind the idiom straw vote.