Consent is usually for phone calls and amounts to notification (hang up if you don't consent). In public states like MA recording in public is mostly about awareness and not consent. If you have a giant camera recording things they won't succeed in prosecuting you for secretly taping audio.
Given the realities of traffic stops, I think it is entirely to be expected that police will read bumper stickers when running plates etc. It gives fair notice. Every officer will read it. And one can verify at the beginning of the conversation.
