Reputedly, all LAPD officers and some civilian employees are issued a master key that opens the back door to any LAPD station house. These are called '999' keys.
The idea of an angel/demon cutting off their wings, dying in human form and thus returning to heaven/hell (as alluded to by Amenadiel) is also a major plot point in the religious comic fantasy film 'Dogma.'
999 is the emergency services number in the United Kingdom.
The title is said by Lucifer.