Incident number

An incident number is a reference number used by police when officially recording a particular crime.

When investigating Bilius Finbok's disappearance, Ministry of Magic investigators Mathilda Grimblehawk and her partner posed as a Muggle police officer and inspector, respectively, when questioning Finbok's Muggle neighbour Sara Farnham. When they had collected sufficient testimony, Farnham asked for the case's incident number; a confused Grimblehawk tentatively answered "four" before leaving in a hurry.