Just about all Fed radios have the option to switch back and forth between encrypted and in the clear. If they were in the clear one moment and then switch to encryption, it's most likely because they chose to communicate something in a sensitive nature. An active FED agent currently on the job is not going to take the time to suddenly log on to radio ref just to find out if someone is monitoring them. Bologna!
In regards to PD, such as Plainfield Detectives, Anyone could find out which frequency they were using by going to the FCC db, so what folks post on here doesn't make one bit a difference...hence not the reason they went encrypted. Eventually all PD's will go encrypted either way despite what is posted here, unless a miracle new future law prohibits it.
Fyi...from what I understand, ALL NJICS radios have the ability to go encrypted whenever they want and they ALL know that many hobbyists enjoy monitoring them already regardless of these forums.