Which of the following categories does NOT belong to police/suspect encounters?

The category that does not belong to police/suspect encounters is investigations. While investigations may involve interactions with suspects and witnesses, they are primarily a process of gathering information and evidence to solve a crime, rather than a direct encounter type like voluntary encounters, detentions, or arrests.

Voluntary encounters are interactions initiated by police where the individual is free to leave at any time, detentions involve a temporary and limited restriction of a person's freedom for police investigation based on reasonable suspicion, and arrests are when a person is taken into custody based on probable cause that they have committed a crime. Investigations, on the other hand, encompass a broader range of activities that may not directly involve the coercive aspects of police-suspect encounters, focusing instead on the methods utilized by law enforcement to gather and analyze information related to crimes.