What is the test for making a declaration of a mistrial on the basis of witness testimony?

[714 P.2d 1260] In People v. Huff, supra, 255 Cal.App.2d 443, 63 Cal.Rptr. 317, for example, the declaration of a mistrial on the basis of an officer's testimony that he had observed the defendant talking to two jurors during a recess was held to be an abuse of discretion. The appellate court in that case stated: "[T]he circumstances called for a hearing to determine what had actually taken place. Particularly apt here is the language of the court in Mitchell v. Superior Court (1962) 207 Cal.App.2d 643, 650, 24 Cal.Rptr. 671: 'A jury should not be discharged in the absence of legal necessity, and it appears that in this case no attempt was made to determine that question and the decision was made by a judge not in a position to make such observation as would enable him to arrive at a proper decision.' " (Id., 255 Cal.App.2d at p. 448, 63 Cal.Rptr. 317.)

"A 'good cause' determination in this context is one calling for the exercise of the court's discretion [citations], and if there is any substantial evidence supporting the decision, it will be upheld on appeal. [Citation.]" (People v. VanHouten (1980) 113 Cal.App.3d 280, 288, 170 Cal.Rptr. 189.) But once the court is put on notice of the possibility a juror is subject to improper influences it is the court's duty to make whatever inquiry is reasonably necessary to determine if the juror should be discharged and failure to make this inquiry must be regarded as error. (People v. McNeal, supra, 90 Cal.App.3d 830, 838-840, 153 Cal.Rptr. 706.)