Motion Detector Light-Intermittent?

We have three InteriorFollower motion detectors installed on our system. The outside appearance of two of them are different than the third. The third's red light frequently lights when we walk through the room with the system not armed. This light is reassuring because it confirms that the motion sensor is working as intended. However, the other two do not light up when there is movement in front of them although they did for several hours after the technician recently worked on the system. All three sensors are showing "good" connection and battery indications. I hope that there is another way to be confident the motion sensors are actually working without setting the alarm to "away" and then walking into each zone.