What is the test for proving a witness unavailable?

In order to demonstrate that a witness is unavailable, the proponent of the hearsay testimony must establish he has "'exercised reasonable diligence but has been unable to procure [the witness's] attendance by the court's process.' [Citation.]" (People v. Herrera (2010) 49 Cal.4th 613, 622.) Factors relevant to the due diligence inquiry include the timeliness of the search, the importance of the testimony, and whether any leads to the witness's whereabouts were competently investigated. (Id. at p. 622.) Other factors include whether the proponent had reason to believe the witness would appear willingly before beginning to serve him and whether the witness would have been produced had additional efforts been made. (People v. Linder (1971) 5 Cal.3d 342, 347.) There is no requirement that a prosecutor "keep periodic tabs on every material witness in a criminal case." (People v. Friend (2009) 47 Cal.4th 1, 68.)

Due diligence may be shown even where the proponent begins looking for a witness immediately prior to trial. (See, e.g., People v. Saucedo (1995) 33 Cal.App.4th

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