Police, Emergency Services, Hospitals and Entertainment Venues etc. where the Public are together in large numbers, use Some Secret Codes to pass information between The Employees.
These are meant to be a Secret as they don't want to Alarm the Non-Staff Members or Alert someone (like a Thief) to the fact that they have been noticed. But there are a Number that are nearly Universal in Application. This is a List of Ten Secret Codes that may prove useful to you in future or at least dispel any curiosity you may have if you hear them.
