RED FLAG (SIGNAL)

'"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.
Also, there is an element of some letter box construction, a semaphore arm that is raised to indicate to the postman that there is outgoing mail in the letter box.
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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See also

See also

★ Red

★ Racing flags