When a police officer searches a knapsack of a suspect in a search for drugs, what is the limitation of the search warrant?

When Officer Denney opened the side pocket of the knapsack he found a frosted, opaque plastic bottle and a pair of envelopes. Because of the translucent but nontransparent nature of the bottle, the People could not rely on the exception to the warrant requirement for objects or contraband found in plain view. (Compare People v. Block (1971) 6 Cal.3d 239, 103 Cal.Rptr. 281, 499 P.2d 961 (contraband found in clear plastic vial during valid search of premises for additional suspects).)