I enjoyed a nice US Secret Service monitoring treat at UNH in Durham NH back in 1988 before, during, and after a debate by the Democratic presidential candidates which was held at UNH. There was LOTS of Secret Service activity on their Tango and Charlie channels in the days before, during, and after the event. On the day of the event, because there were so many protectors needed for so many protectees, there was not enough "new" Motorola analog radios w/ENC to go around. The Secret Service had to break out their stash of ancient Motorola analog bricks having NO encryption. Having to dig out and use the old radios was specifically mentioned over the air. The end result was that more than 90% of comms by the Secret Service was in the clear and easily monitorable.
More recently (1990s) during a POTUS visit in Manchester NH, USSS comms were fully encrypted and the PD conveniently provided a way to listen. As status updates were given by an officer, a USSS portable was heard chattering in the background and the officer was heard repeating everything that was said almost verbatim.
A lot can be learned by exploiting the "holes" in the system.