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 etcExamples from the Corpusred flag• Volcanologists 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.