a warning to be on the lookout for a missing child, sent out by the police over an emergency system that uses radio and TV broadcasts, roadside electronic displays, etc.
after Amber Hagerman (1986-96), child abducted and murdered
See Amber Alert in American Heritage Dictionary 4
A message that conveys information about a recently missing or abducted person, usually displayed on electronic signs positioned along roadways and broadcast by mass media, intended to enlist the public's help in finding the abducted person and often in catching the abductor.
Origin: After Amber Hagerman (1987-1996), American child kidnapped and murdered in Texas