A red flag is a warning. It means be careful. Proceed with caution. Your date says all his exes were jealous or crazy. That's a red flag. Your new girlfriend hangs out alone with her ex a lot. That's a red flag.
Separately, there's the actual horrible behavior or action that the red flags were trying to warn you about.
I see threads like "My girlfriend always yells at waiters and fast food workers and treats them like shit" or "my boyfriend punches holes in the wall" and very often, there's comments along the lines of "Jesus, how many red flags do you need before you get out of dodge." Or there's an AskReddit thread like, "What are some red flags you've ignored in a relationship?" And many answers mention alcoholism, drug addiction, cheating, physical violence.
Those aren't red flags. There's the smoke detector going off, and then there is the actual fire. There is no catchy term for it but people still call them red flags and no one ever corrects it.