First and foremost they would have to know that Jill knew this man. She certainly did not advertise all her brief flings and one night stands, even to her friends, and her friends might be reluctant to tell the investigators about them, wanting to protect her reputation.
There is also a possibility this man is someone who appeared on the fringes of Jill's life, someone she met briefly at work, someone who was performing some service for her, someone introduced to her on some social occasion, or tried to flirt with her one time in a bar. Someone who did not have real contact with her, but developed unhealthy obsession. Such a person would not get on the police radar that easily.