The full form of APB is An all-Points Bulletin.

a general bulletin broadcast to alert law-enforcement officers over a wide area that someone (such as a suspect) or something (such as a vehicle) is being actively sought in connection with a crime

An all-points bulletin (APB) is a broadcast issued from any American or Canadian law enforcement agency to its personnel, or to other law enforcement agencies. It typically contains information about a wanted suspect who is to be arrested or a person of interest, for whom law enforcement officers are to look. They are usually dangerous or missing persons. As used by American police, the term dates to at least 1947.^[1]

An all-points bulletin can also be known as a BOLO or BOL, which stands for “be on (the) look-out”. Such an alert may also be called a lookout or ATL (“attempt to locate”).