From one dope bust case:
Although Greenwood had hidden the trash from view by putting in opaque plastic bags and expected it to be on the street only a short time before it would be taken to the dump, the Court believed it to be “common knowledge” that garbage at the side of the street is “readily accessible to animals, children, scavengers, snoops, and other members of the public.” Moreover, Greenwood had left the trash there expressly so that the trash collector, a stranger, could take it. Quoting Katz v. United States, the court concluded that "[w]hat a person knowingly exposes to the public, even in his own home or office, is not a subject of Fourth Amendment protection."