1. a notification that there is a problem, something that draws attention to a problem
You'll raise a red flag with your bad behaviour
A 'red flag' is a warning or an indication that an important issue needs your attention. People generally raise a red flag when there is an issue or they can visualize an upcoming issue ahead of time. You can also 'red flag an issue' or you can say, "This issue needs to be red-flagged". A 'red flag' is something that is considered 'a warning sign' or an indication that something is wrong. This is an idiom used in social and professional contexts.