Have you tried searching the basic channel plans for Federal users? Not all channels are active like a local police dept, so you usually only hear activity when something is on-going.
Try 137-144 , 148-150.7500, 162-174, 406-420 for a basic search.
Make Notes [Tones etc] and post back what you hear in a few weeks.
As for the NAY sayers, NOT all Federal traffic is Encrypted. There is still plenty going on for US to listen to. Just a little patience is needed.
What are the frequencies used by the FBI, US Marshalls, etc. in Union county?
Any help appreciated. Thanks.