Sometimes they are activated when the lights are activated
Usually they are recording all the time, with a ‘marker’ on the tape when lights are turned on, thereby marking the recording to the relevant part of a stop. . .kind of like having a huge bound notebook documenting everything you did all day and when something happens, when you go to court and the entire book is entered into evidence. Talk about noise and distraction. Anyway, unless you can mark/extract the exact data from the exact time, the entire recording is placed in evidence. . .not a problem but with defense lawyers the way they are, what is the point of putting into evidence a recording of a previous shift where the officer was having lunch in his car? With the marker the recording is identified and can be extracted for court action.