on the alert
Particularly watchful, attentive, or prepared (for something). Security personnel are on the alert for anyone carrying suspicious-looking packages or behaving strangely. We've been on the alert ever since we heard there had been a string of burglaries in our part of town.
on the alert (for someone or something)
watchful and attentive for someone or something. Be on the alert for pickpockets. You should be on the alert when you cross the street in heavy traffic.
on the alert
Watchful, fully prepared, as in The inspectors are always on the alert for a manufacturing error. [Late 1700s]
on the alert
Watchful and prepared for danger, emergency, or opportunity: bird watchers on the alert for a rare species.