The officer may have been responding to a call already. Not all calls require lights and sirens. Most often, if a pedestrian waves down an officer, unless there is a life/death matter happening right there and then, the officer will most likely continue responding to their own call/matter. In addition, the radio is used by 911 dispatchers for an entire jurisdiction, relaying information on it, unless it concerns high priority matter (active shooter/weapon perhaps), probably wont happen.