The expectation of privacy test, originated from Katz v. United States is a key component of Fourth Amendment analysis. The Fourth Amendment protects people from warrantless searches of places or seizures of persons or objects, in which they have an subjective expectation of privacy that is deemed reasonable in public norms. The reasonableness standard is construed upon the totality of circumstances on a case-by-case basis. The person’s precautions taken to exclude others’ access are strong indicators to the expectation of privacy and might be taken into consideration by the court.
The legitimate expectation of privacy must have a independent source outside Fourth Amendment. For example, private homes are at the core of Fourth Amendment protection subject to a few exceptions, as they are closely associated with the ownership interest in property law.
On the other hand, a warrantless seizure of abandoned property usually does not violate the Fourth Amendment. Temporary residents on public property without permit and in violation of law do not have legitimate expectation of privacy either. Moreover, the Fourth Amendment protection does not expand to governmental intrusion and information collection conducted upon open fields. Expectation of privacy in an open field is not considered reasonable. There are some exceptions under state laws that grant protection to open fields.
Katz v. United States: http://www.law.cornell.edu/supremecourt/text/389/347