Fourth Amendment: Searches, Seizures, Warrants, and Exceptions

The Fourth Amendment restrains only governmental conduct (including private parties acting as government agents), not purely private searches. The next question is whether a 'search' occurred. Under Katz v. United States, a search happens when the government intrudes upon a subjective expectation of privacy that society recognizes as reasonable — 'the Fourth Amendment protects people, not places.' United States v. Jones added a property-based trigger: physically trespassing on a constitutionally protected area (a car, in Jones, via a GPS tracker) to obtain information is also a search.

There is no reasonable expectation of privacy in things knowingly exposed to the public: the open fields beyond the home's curtilage (Oliver), garbage left at the curb (Greenwood), bank records and numbers dialed (the third-party doctrine of Miller and Smith), and what is visible from public navigable airspace. But the Court has limited the third-party doctrine: Carpenter v. United States held that accessing seven days of historical cell-site location information is a search requiring a warrant.

The home and its curtilage receive the greatest protection; Kyllo held that using a thermal-imaging device to detect heat inside a home is a search, and Florida v. Jardines held that bringing a drug dog onto the front porch is a trespassory search.

The default rule is that a search or seizure is unreasonable unless conducted pursuant to a warrant. A valid warrant has three requirements: (1) probable cause — a fair probability that contraband or evidence of a crime will be found in the place to be searched, which may rest on an informant's tip evaluated under the Gates 'totality of the circumstances' test; (2) particularity — the warrant must describe with particularity the place to be searched and the persons or things to be seized; and (3) a neutral and detached magistrate must issue it (a magistrate who is paid per warrant, or who leads the search, is not neutral).

Police executing a warrant must generally knock and announce, though a no-knock entry is permitted where announcement would be dangerous or futile (and a knock-and-announce violation does not trigger exclusion under Hudson v. Michigan).

If a warrant is later found defective, the good-faith exception of United States v. Leon usually preserves the evidence where officers reasonably relied on a facially valid warrant — unless the affidavit was so lacking in probable cause that reliance was unreasonable, the magistrate was misled by a knowing or reckless falsehood, the magistrate abandoned his neutral role, or the warrant was facially deficient in particularity.

Most fact patterns turn on a warrant exception. (1) Search incident to a lawful arrest: police may search the arrestee's person and the area within his immediate control (the wingspan); for a vehicle occupant's arrest, Arizona v. Gant limits the search of the passenger compartment to when the arrestee is unsecured and within reaching distance OR it is reasonable to believe evidence of the crime of arrest is in the vehicle; Riley v. California requires a warrant to search a cell phone seized incident to arrest.

(2) Automobile exception: with probable cause that a vehicle contains contraband or evidence, police may search the entire vehicle and any container within it that could hold the object of the search, without a warrant, because of the vehicle's mobility and reduced expectation of privacy. (3) Plain view: an officer lawfully present may seize an item whose incriminating nature is immediately apparent. (4) Consent: a voluntary consent by someone with actual or apparent authority validates a search; a physically present co-occupant's objection overrides another's consent (Georgia v. Randolph).

(5) Terry stop-and-frisk: a brief seizure on reasonable, articulable suspicion of criminal activity, plus a protective frisk (pat-down of outer clothing) for weapons on reasonable suspicion the suspect is armed and dangerous. (6) Exigent circumstances: hot pursuit of a fleeing felon, imminent destruction of evanescent evidence, or the emergency-aid doctrine permits warrantless entry. Administrative/special-needs searches (border, checkpoints, schools, probationers) round out the exceptions.

A complete Fourth Amendment analysis must separate searches from seizures of the person and master the special-needs cluster. A seizure of the person occurs when, by means of physical force or a show of authority to which the suspect submits, a reasonable person would not feel free to leave or to terminate the encounter (United States v. Mendenhall; California v. Hodari D. requires either physical force or submission to authority, so a suspect who flees during a show of authority is not seized until caught). A consensual encounter — an officer approaching and asking questions in public — is not a seizure and requires no suspicion at all.

A Terry stop is a seizure requiring reasonable suspicion; an arrest is a seizure requiring probable cause, and a warrant is generally not required to arrest in a public place, though Payton v. New York requires an arrest warrant to arrest a person in his own home absent exigency or consent.

The special-needs and administrative exceptions dispense with individualized suspicion where the primary purpose is something other than ordinary crime control: border searches (routine searches at the international border need no suspicion), fixed sobriety and immigration checkpoints (Sitz;

Martinez-Fuerte), inventory searches of impounded vehicles and arrestees' effects conducted under standardized procedures, administrative inspections of closely regulated industries, searches of students on reasonable suspicion in public schools (T.L.O.), suspicionless drug testing of certain railroad and customs employees and student athletes, and searches of probationers and parolees on reduced suspicion (and, for parolees, suspicionless searches under Samson v. California).

But a checkpoint whose primary purpose is general crime control or drug interdiction is unconstitutional (City of Indianapolis v. Edmond), a heavily tested trap. Several recurring traps deserve emphasis. First, standing: even a clearly illegal search yields no remedy unless the defendant's own reasonable expectation of privacy was invaded — a point developed fully in the exclusionary-rule section but always paired with the search analysis.

Second, the automobile exception does not require exigency beyond the vehicle's inherent mobility, and it extends to any container within the car that could hold the object of the search, including a passenger's belongings (Wyoming v. Houghton). Third, plain view requires that the officer be lawfully positioned AND that the incriminating nature be immediately apparent without further searching (Arizona v. Hicks barred moving stereo equipment to read serial numbers).

Fourth, consent must be voluntary under the totality of the circumstances, need not be preceded by a warning that the person may refuse (Schneckloth v. Bustamonte), and is limited in scope to what a reasonable person would understand the consent to cover (Florida v. Jimeno). Fifth, technology cases trend toward protecting privacy: Riley requires a warrant for cell-phone data incident to arrest, Carpenter requires a warrant for extended historical cell-site location records, and Kyllo and Jardines protect the home and its curtilage from sense-enhancing and trespassory canine investigation.

California, by virtue of Proposition 8 ('Truth-in-Evidence,' 1982), conforms its exclusionary remedy to federal Fourth Amendment standards, so the federal doctrine generally governs suppression in California courts and divergent state constitutional protections rarely yield broader exclusion. The disciplined approach: classify the police conduct as a search, a seizure, or neither; identify the privacy or liberty interest invaded; demand a warrant; and, if none exists, test each exception against its precise trigger before reaching reasonableness of scope.

Levels of Suspicion and What They Authorize