along with using numbers from the criminal code they will also use their car numbers instead of their names, for instance the Chief of Police for the City near hear goes by "34" which is short for 13-34 which is his car. They also use a number instead of the City names, for instance the City I live in is referred to as 401, because it's where the County Court, Sheriff's Department etc. are all located at. The numbers all go up from here so the next closest city or town is call 402. They do this so they don't have a bunch of bored people with scanners following them around everywhere they go.Nothing against people with scanners but some just don't know how to stay out of the way.
Of course they also use 10 codes and signal codes, like every other department. If you're never hearing any 10 codes then the department you're listening to may have just gotten in the habit of dropping the 10 and just using the last set of numbers in the codes, OR they are using their own set of codes. There is no universal set of 10 codes and they vary wildly from one department to another, so there's no real way to know for sure what that department is using.
Edit: Jay makes a very good point. Departments are supposed to be slowly phasing out 10 codes and going to plain speech. One of the big reasons behind that decision was that there was very often a lot of confusion being caused because there was not one set standard for 10 codes. The list that Silvercharm has might be accurate in some places, but around my area about 1/3 of those are wrong.
10yrs Living with and working closely with Law Enforcement