My first experience with the term "red flags" was in connection with the IRS, the individual incometax returns that all of us have to submit to the government, beginning early in February or whenever we get our W2s, or our wage slips telling how much we earned for the year, and other forms giving basically the same information, like reporting the amount of money one collected from unemployment compensation, or social security, and others that I have forgotten, or that I am not aware of.
A red flag for the IRS is if someones income reporting suddenly drops without any good reason. A large change in any kind of reporting information without good reason. Say a person suddendly has large expendatures and deductions after not having them the year before. A red flag is basically any change that makes the IRS suspicious of the honestly of a person's reporting on the individual income tax return.
An example of this is a minister friend of mine from the Assembly of God church. He gave so much to missions that it made the IRS suspicious of his honestly, so they called him in for an audit of his tax return. He really did give that large amount to missions, but the IRS could not believe that anyone would give as much to missions as he did. He didn't get into trouble as he was honest, but it just caused him a lot of trouble to have to meet with them and furnish proof that he had given that amount. His reporting of his missions giving was a red flag to the IRS, so any time they get a red flag a person is apt to get audited by the IRS.
Red flags in connection with what we are talking about here would be any action/actions of an individual that would be suspicious. For instance I have heard of bfs/gfs calling each other, one calling the other to find out if that person was where they were supposed to be. In other words if the guy from USA called his girlfriend in the middle of her night when she was supposed to be sleeping and she was not at home, it might be a red flag telling him that she might not be where she was supposed to be. In the case of the above person posting he felt that the way this young lady treated him was a red flag telling him that there might be something not quite right going on. She might have another boyfriend on the side, or she might just not like him, or there might be something even worse going on.
She is not telling him exactly how she feels, so he is coming to his own conclusions which may or may not be correct. But he feels like he has seen enough indications that she is not what she should be, especially to him, that it will not work for him. She will not furnish the document saying that she is not married. He feels that is a red flag. When she doesn not say "I love you" first he sees that as a red flag. He sees her snapping at him in an unfriendly manner as being a red flag. He feels that her cold treatment of him is a red flag. Because of those things he makes other assumptions about her that may not be true. She may just not like him, and is hesitant to tell him that. Then he said that she messed with him facebook, changing the password on him. She also hacked into his e-mail. Those actions indicate that she still has feelings for him, or she is really angry with him, so who knows. She is not telling him, and he is coming to his own conclusions which may or may not be true, and she may just have a problem with him, and not really have problems herself. I don't know the answer. In any case he has had a bad experience, and she is not very happy either.