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.)
But, as a matter of logistics, it can be difficult to shield a backyard from outside viewers. And the public's ability to see into your yard is a factor supporting officers' legal authority to search it without a warrant. For example, a police officer who is in a publicly accessible place but can easily see something incriminating in your yard can probably cite that evidence in support of a warrant to search your home (and yard).
Consider the following example, which illustrates the limits on the extent to which the government can snoop around your property.
Police officers receive a tip from a confidential informant that Emilio has just stolen some anhydrous ammonia, which is often used to manufacture methamphetamine. They discover that there is a warrant outstanding for his arrest, so they go looking for him. They eventually go to the home of his known associate, Jesse, late at night.
When the officers arrive at Jesse's home, they see Emilio's car in the driveway. They get out of their patrol car and walk down the side of the house, all away around to the back. There are no lights on, so the officers use flashlights. They notice several "no trespassing" signs on the property, which is bordered by a wooded area (other than the front of the house, which has a road leading to it). There's no fence.
The officers observe a shed and a mobile home in the backyard. They begin to walk between the mobile home and the shed, when suddenly the shed door swings open and Jesse runs out. The officers order him to the ground, and he complies. The door to the shed is now open such that the officers can see Emilio, various jars and glasses containing liquids, and a hot plate inside. The officers order Emilio out. After he exits the shed, they take a glance inside to see if anyone is still in it. They see a methamphetamine manufacturing setup and take photographs of it. A couple officers leave to get a warrant, then come back with it, at which point all the officers search the entire residence. They find more incriminating evidence.
In his prosecution for methamphetamine manufacturing, Jesse moves to suppress all the evidence the police officers gathered. The court grants the motion on the following grounds:
Jesse therefore had a reasonable expectation of privacy in his backyard. (State v. Kruse, 306 S.W.3d 603, 611 (Mo. Ct. App. 2010).)