The phrase "warning bells" refers to a metaphorical sound that indicates caution or danger ahead. It implies that there are signals or signs alerting us to be careful or stay vigilant about a certain situation. Full definition
That provides a reminder that good quality research can provide the necessary warning bell for trouble when CEOs change. (afr.com)
Of course, investors should always have a plan to take profits, limit losses, and unwind positions quickly when warning bells go off. (dailytradealert.com)
Having said that, if I get warning bells about a person, I am likely to pass. (annakatmore.com)