keep (one's) weather eye open
An instruction to monitor someone or something, often because the person or thing in question may turn out to be dangerous. I think Tom's a thief, so keep your weather eye open around him. He needs to keep his weather eye open with this storm that's brewing, in case he has to evacuate.
keep one's weather eye open
Fig. to watch for something (to happen); to be on the alert (for something); to be on guard. Some trouble is brewing. Keep your weather eye open. Try to be more alert. Learn to keep your weather eye open.