One thing to keep in mind (based on my experience) is that the feds generally work weekday business hours. Obviously that's not 100% true but if you're not monitoring during weekday business hours then you're likely missing a good chunk of federal activity on average. I recommend setting a scanner to auto-record. Also, lots of raids/warrants are conducted during the early AM hours. Also consider the agency, e.g., USPIS can more active in the overnight hours. You may be surprised what you find.
With respect to encryption, even the "mostly encrypted" agencies will go unencrypted from time to time and if you're not always listening then you can't hear it. Here are my observations by agency (your mileage may vary).
These are "mostly" encrypted but not always (I've heard unencrypted comms on all of them): ATFE, USCG (on the non-marine frequencies), FBI, Marshals, USPIS, USSS.
These are not encrypted at all or are frequently unencrypted: BOP, CBP, ICE, DEA, Department of State, FAA, FPS, Federal Reserve Bank, USPS (postal operations), Veterans Affairs.
Also don't forget the federal aviation frequencies which are not encrypted: 120.375, 136.275, 136.375, 350.025, etc.
Another example: the recent visit of the President of Liberia to Houston had unencrypted comms on USSS Charlie (simplex so you have to be reasonably close).