keep a weather eye open idiom
(also keep a weather eye out)随时保持警惕
to always be paying attention so that you notice if anything happens
His bodyguard kept a weather eye open from across the room. 他的保镖在房间的另一头时刻保持警惕。
He has informants all over the town who keep a weather eye out for him. 他在全镇都布有眼线，为他密切关注情况变化。