A scanner (also known as an auspex or surveyor) is used to detect hidden enemy troops. If enemy infiltrators set up within 4D6" of a model with a scanner then the model is allowed to take a 'free' shot at them (or sound the alarm in a Raid scenario). If the model is part of a unit, the whole unit may shoot. These shots are taken before the battle begins and may cause the infiltrators to fall back.
Scanner
Source: Codex: Imperial Guard (1999), p. 8