In what circumstances will a police officer conduct an in-custody interrogation?

Our case is in many ways on all fours with People v. Abbott, Supra, 3 Cal.App.3d 966, 84 Cal.Rptr. 40. There, an informer, who probably was a 'citizen-informer' but whose reliability had not been established, told a police officer that Abbott had marijuana in his coat pocket. The informer pointed out Abbott and then left. The officer approached Abbott and began to ask him questions of an unrelated nature solely in order to delay the suspect until the officer's partner could arrive. What his partner did arrive, the officer asked the suspect what he had in his coat pocket, and Abbott finally pulled out the marijuana and said that it was 'grass' (i.e., marijuana). The court noted that the 'evidence is uncontradicted that at the time the arresting officer approached and began to interrogate (Abbott), the investigation was not a general inquiry . . .; suspicion . . . had focused specifically upon (Abbott). It is also clear that the question asked . . . was intended to elicit an incriminating statement. Moreover, it is uncontradicted that the arresting officer's purpose in engaging (Abbott) in conversation was to prevent his leaving . . .. Therefore, . . . the officer's question constituted an in-custody interrogation.' (Id., pp. 969--970, 84 Cal.Rptr. p. 42.)