A more thorough explanation:

The Open Field Doctrine is a legal principle that states that the Fourth Amendment protection does not apply to open fields. This means that the government can collect information and intrude upon open fields without a warrant or violating the Fourth Amendment.

Even if there are fences or "no trespassing" signs around the field, the expectation of privacy in an open field is not considered reasonable or legitimate. Open fields may include any unoccupied or undeveloped area outside the curtilage.

Some states reject the Open Field Doctrine and confirm protection under state law of privately owned open lands from warrantless searches and seizures, where a reasonable person should know that the occupant intends to exclude public and precautions have been taken.

For example, in State v. Dixon, the Supreme Court of Oregon held that Article I, Section 9 of the Oregon Constitution, which is the search and seizure provision, applies to privately owned lands outside the curtilage of a residence. This means that the police cannot search or seize items from a privately owned open field without a warrant or probable cause, as long as the owner has taken reasonable steps to keep the area private.