From Longman Dictionary of Contemporary Englishred alertˌred aˈlert noun [countable usually singular, uncountable] WARNa warning that there is very great dangerbe (put/placed) on red alert All the hospitals are on red alert.
Examples from the Corpus
red alert• It was the normal state of red alert, panic, and disarray in the Bronx County Building.• The National Health Service was put on red alert.• Frank had to leave-he'd burnt so many bridges Blue Watch were on red alert.• If we bypass the decision we simply move into autopilot and the red alert comes into play.be (put/placed) on red alert• The National Health Service was put on red alert.• Perhaps that was why every cell of her normally controlled body seemed to be on red alert in Rune Christensen's presence.• Frank had to leave-he'd burnt so many bridges Blue Watch were on red alert. 