Data Scanning and the Fourth Amendment

A crucial question of Fourth Amendment law has recently divided courts: When government agents conduct a digital scan through a massive database, how much of a “search” occurs? The issue can pop up in diverse contexts including geofence warrants, reverse keyword searches, tower dumps, and Internet pen registers. When a government agent runs a filter through a massive database, resulting in a list of hits, is the scale of any Fourth Amendment search determined by the size of the database, the filter setting, or the filter output? Fourth Amendment law is closely attuned to the scale of a search. No search means no oversight, small searches ordinarily require warrants, and limitless searches are categorically unconstitutional. But how broad is a data scan? This Article argues that that Fourth Amendment implications of data scans should be measured primarily by filter settings. Whether a search occurs, and how far it extends, should be based on what information is exposed to human observation. This standard demands a contextual analysis of what the output reveals about the dataset based on the filter setting. The proper question is what information is expressly or implicitly exposed, not how much raw data passes through the filter or what the raw data output says expressly. The implications of this approach are then evaluated for a range of important applications, among them geofence warrants, reverse keyword searches, tower dumps, and artificial intelligence queries.