Since federal agencies have started using IMBE P-25 digital radios, and scanner listeners have had access to scanners that will display the P-25 NAC (Network Access Code), there has been an ongoing effort to discover any patterns to the use of any particular NACs by different federal agencies.
While much of federal radio traffic is enrypted, the identification of the user by the NAC can be a big help when finding new and encrypted frequencies.
So far, there have been only a few federal agencies that appear to be using a particular NAC for all their frequencies. Here is a list if what has been confirmed so far:
BATF uses N650, N651, N652 and N653.
DEA uses N156.
FBI often uses N167, but has been seen using other NAC's on their frequencies. Why is still a mystery.
SECRET SERVICE is using N001 on their simplex and repeater output channels, but so are other agencies (TSA, CBP Border Patrol).
STATE DEPARTMENT Security Service appears to be using N0F0.
It now appears that the US Postal Inspectors are using a common NAC on their new P-25 repeaters. That NAC is N482, so if you are serching around the federal UHF (406-420 MHz) band and come across an encrypted frequency using a NAC of 482, that will most likely be the Postal Inspectors.
More coming as we figure these things out...