You can turn it on any time, and you have to do maintenance even when in port. However, we made a habit of manning the console from the time the Sea and Anchor detail was set, onward. If we were away from the pier the console was manned. Of course, that was a while ago, peak of the Cold War and all.
In order to listen to police (FM voice) you had to slope detect the audio.
Here's another receiver in my collection - an Ailtech NM37/57 field strength meter - 30MHz to 1GHz in 8 bands and again with sensitivity down to -120dBm. Every band has it's own tuner and it will scan the whole band or just a segment - with a scope connected to the output it will display the signal amplitudes. We used these on our calibration aircraft in the 80's but the gummint closed us down in the 90's. This appeared on our local auction website with an opening bid of NZ$100, say US$60. I couldn't turn it down, could I?...... The pic shows the local FM BC band signals.