English version

red flag

From Longman Dictionary of Contemporary Englishred flagˌred ˈflag noun [countable]  American English something that shows or warns you that something might be wrong, illegal etc
Examples from the Corpus
red flagVolcanologists now recognize these oscillations as a red flag that a volcano is entering a dangerous phase.It was a red flag, a symbol of an anti-education attitude.All we needed was a red flag to warn the locals that we might be about to frighten the horses.
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