She said that to the two men, saying that her Dad had told her Never trust a man who doesn't tell you his surname, when they introduced themselves just using supposed first names.
Never trust a man who doesn't tell you his surname.
Said by the character of Polly in ITV police series The Bill when undercover as a purported new client while checking out two men who were suspected of charging young women money for taking fashion photographs of them, but were not using film and were scarpering with the money.