Keep a weather eye on somebody/something
Definition 1 of 1
Pro Tip 1/3Metaphorical Use
The phrase is often used metaphorically, suggesting careful attention without implying actual weather. 
As the debate heated up, the moderator kept a weather eye on both speakers.
Pro Tip 2/3Vigilance Required
It implies a need for persistent vigilance, not just a single check. 
With the recent increase in cyber attacks, IT managers must keep a weather eye on network security.
Pro Tip 3/3Nautical Origin
The phrase originates from sailing, hinting that one should be ready to respond to changes. 
As they approached the stormy cape, the captain kept a weather eye on the swells.