What is the Single-Purpose Container exception to 4th Amendment warrant requirement?

The “single-purpose container” exception to the warrant requirement originated in the United States Supreme Court’s decision in Arkansas v. Sanders, 442 U.S. 753, 99 S.Ct. 2586, 61 L.Ed.2d 235 (1979), overruled on other grounds by California v. Acevedo, 500 U.S. 565, 111 S.Ct. 1982, 114 L.Ed.2d 619 (1991). The central question in Sanders was “whether, in the absence of exigent circumstances, police are required to obtain a warrant before searching luggage taken from an automobile properly stopped and searched for contraband.” Id. at 754, 99 S.Ct. 2586. The Court answered this question in the affirmative, but declared:

Not all containers and packages found by police during the course of a search will deserve the full protection of the Fourth Amendment. Thus, some containers (for example a kit of burglar tools or a gun case) by their very nature cannot support any reasonable expectation of privacy because their contents can be inferred from their outward appearance.
Id. at 764 n. 13, 99 S.Ct. 2586.

In Robbins v. California, a plurality of four justices elaborated on the “single-purpose container” exception, explaining that the exception is:

little more than another variation of the “plain view” exception,[7] since, if the distinctive configuration of a container proclaims its contents, the contents cannot fairly be said to have been removed from a searching officer’s view. The same would be true, of course, if the container were transparent, or otherwise clearly revealed its contents. In short, the negative implication of footnote 13 of the Sanders opinion is that, unless the container is such that its contents may be said to be in plain view, those contents are fully protected by the Fourth Amendment.
453 U.S. 420, 427, 101 S.Ct. 2841, 69 L.Ed.2d 744 (1981) (plurality opinion), overruled on other grounds by United States v. Ross, 456 U.S. 798, 102 S.Ct. 801*801 2157, 72 L.Ed.2d 572 (1982).

Full case here: US v. Gust, 405 F. 3d 797 – Court of Appeals, 9th Circuit 2005, https://scholar.google.com/scholar_case?case=13976317218493731054&hl=en&as_sdt=6&as_vis=1&oi=scholarr#p807