If the address of a person or entity on whom citation is required to be served cannot be ascertained, citation must be served on the person or entity by publication in the county in which the proceeding to declare heirship is commenced and in the county of the last residence of the decedent who is the subject of the proceeding, if that residence was in a county other than the county in which the proceeding is commenced. To determine whether a decedent has any other heirs, citation must be served on unknown heirs by publication in the manner provided by this section.