The red light is referred to as a danger signal.

危险信号。

The sun was setting when the boy's heart sounded a danger signal.

太阳开始下沉的时候，男孩的心发出了危险的信号。

Party conduct is frame - up, it is a danger signal of political qualitative change.

党风不正, 是政党质变的一个危险信号.