be on the lookout for somebody

Từ điển Oxford Advanced Learner 8th

be on the ˈlookout (for sb/sth) | keep a ˈlookout (for sb/sth) idiom
(informal)to watch carefully for sb/sth in order to avoid danger, etc. or in order to find sth you want
•The public should be on the lookout for symptoms of the disease.
Main entry: ↑lookout_idiom