Red flag
A 'red flag' is a term used to describe when somebody has a feeling that they are potentially dealing with a manipulative and/or possibly psychotic or sociopathic individual. It's usually used to explain a feeling after the fact.
He didn't like Louis Tomlinson, I should've seen that red flag.
He dislikes Louis Tomlinson, you can't ignore that red flag, you have to drop him.
Stream defenceless, that is not red flag