When is a police officer required to obtain a warrant to seize marijuana in plain sight?

In People v. Kampmann (1968) 258 Cal.App.2d 529, 65 Cal.Rptr. 798, the court observed, "Where a lawful entry has occurred the seizure of contraband observed in plain sight is not unreasonable; on the contrary, it is the lawful duty of the police officer. To require in the present case that one officer go to obtain a warrant while the other remains camped by the marijuana would further no recognizable interest; it would magnify technicality at the expense of reason." (258 Cal.App.2d at p. 533, 65 Cal.Rptr. at p. 802.) When the contraband is concealed subject to constitutional protection and safe from search or seizure, the plain-sight rule cannot apply. Nevertheless the officer should have the alternative of waiting for the return of the suspected owner.