The police term "APB" stands for "all-points bulletin." It is an alert from one police station to all others in the area with instructions about arresting a suspect or suspects.
An APB is sent out when a person of interest is dangerous or if there is a missing person. The term dates back to around 1960. The bulletin typically has information about the suspect that the police officers are looking for. An APB is also known as a BOLO (which stands for "be on the look-out") or as an ATL (which stands for "attempt to locate"). In the United Kingdom, police use the term APW, which stands for "all-points warning."