Almost exactly a year ago, someone unsuccessfully tried to steal Zuckerberg's trash: https://news.ycombinator.com/item?id=16745057 https://news.ycombinator.com/item?id=16723595
The police do this all the time to suspected criminals. They just pick up their trash and if there's evidence there it's admissible in court.
This article specifically disagrees:
"Trash picking is illegal in California — once a bin is rolled out onto the sidewalk the contents are considered the possession of the trash collection company, according to Robert Reed, a spokesman for Recology, the company contracted to collect San Francisco’s garbage. But the law is rarely enforced."