In
Glasgow,
Scotland mob is the name given to
CID or plain-clothes
police officers.
Fairly funny when standing with a group of young people in one of the housing estates and an ordinary looking car pulls up. Someone shouts "MOB!" and the entire group splits up and runs off in different directions. Leaving occupants of said car rather bemused as they were only stopping to ask directions.
For a reason for the name, see pukesicks definition of the word mob above.