"Red flag" is a semi-official term to denote various attention and awareness indicators and signals, both explicit and implicit.
The term and the expression "to raise the red flag" come from various usages of real flags in real life. The semaphore red flag (or red light) on railways means an immediate stop, while a red flag is frequently flown by armed forces to warn the public of live fire exercises in progress, and is sometimes flown by ships carrying munitions.
The term "red flag" is used, e.g., during screening of communications, and refers to specific words or phrases encountered that might indicate relevance to the case. For example, email spam filters make use of such "red flags".
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