1
general::
Watchful, fully prepared, as in The inspectors are always on the alert for a manufacturing error. [Late 1700s]
American Heritage Idioms
2
general::
Phrase(s): 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.
McGrawhill's American Idioms And Phrasal Verbs