I don't think there should be constant activation. Only during confrontation. It'd take the fun out of the job. I wouldn't want all the stupid shit and banter I say between my friends and I on a daily basis to be recorded. When on breaks, just chilling in the car.
I know this is a silly concern, but I'm a sworn protector of the chill.
This can be handled through key management. Give officers an option to switch on a privacy mode through a "key fob" which basically just manages encryption keys. "Privacy mode" time spans are just encrypted with an additional key. If a court proceeding wants something from a "privacy mode" time, they can still request it.
This would prevent officers using this feature to hide improper procedure while also preventing other officers from using the device to eavesdrop.
I know this is a silly concern, but I'm a sworn protector of the chill.