Idaho Code § 6-506: Service by publication.

If a party having a share or interest is unknown, or any one of the known parties reside out of the state, or cannot be found therein, and such fact is made to appear by affidavit, the summons may be served on such absent or unknown party by publication, as in other cases. When publication is made, the summons, as published, must be accompanied by a brief description of the property which is the subject of the action.