Because I just do. I feel it in my gut. His customers turn up dead all the time. Sometimes they get devoured by carniverous asexual bunnies. Sometimes they're eaten by giant eel things that he tries to pass off as mermaids. It happens way too often to just be a coincidence, and I refuse to believe that he just brought her into his shop the night before she mysteriously went missing. You sound like the Chief.
Oh. That explains it then. The dreams aren't... ordinary dreams. They feel like memories and sometimes they give you things. Like cactuses. Or bullet wounds. I'm Leon by the way. Leon Orcot.