Police may seize evidence and contraband that are in plain view, as long as the officer is in a place where he has a legal right to be and the seizure won't involve entering a home (some exceptions apply).
The Fourth Amendment protects your home—including your yard—from warrantless searches in most instances. Your yard is considered “curtilage,” land that surrounds and is associated with a house and is worthy of privacy protection. (Courts determine where curtilage ends on a case-by-case basis.)