[keep] a lookout

to monitor a particular area, situation, or person in order to be aware of any developments, changes, or potential dangers

The lifeguard kept a lookout for swimmers in distress.
During the camping trip, someone needed to keep a lookout for wild animals.
He asked his friend to keep a lookout while he quickly ran into the store.
The security guard kept a lookout for suspicious activity on the surveillance cameras.
In a game of hide and seek, one child is designated to keep a lookout.