Brenda Mayrack never intended to become an unclaimed-property czar. Even among legal specialties, the field is particularly obscure: During law school at the University of Wisconsin, she remembers hearing only a 10-minute lecture introducing the topic at the end of her trusts-and-estates class. But as the director of Delaware’s unclaimed-property office, Mayrack now oversees a fund of $540 million a year, forgotten by people from Paris to San Francisco and then held temporarily by the state.