Inside Job Meaning

(idiomatic) A crime or other illicit action committed by or with the help of someone either employed by the victim or entrusted with access to the victim's affairs and premises.

Example: 1921, Louis Joseph Vance, Alias The Lone Wolf, ch. 10,
  It seems to me it must have been what the police at home call "an inside job"; because whoever it was apparently knew the combination of the safe.