One thing to take into account is that the heatmap colour range is not an absolute scale. It is relative to the total motion for that camera, that date, and the selected time range. What this means is if there is minor motion at 5AM, it may not show up at all if there is significant motion later on in the day and the time range covers a large period of time e.g. 4AM to 11PM. The motion at that time relative to the motion for the total time period is very low.
What you can do is try out different time ranges to discover small changes. In the example I have just given, choosing 1AM to 6AM will provide a much better chance of finding minor activity. This is because the activity at 5AM relative to the other low level of activity at night is more significant.
As has been pointed out, camera placement makes a huge difference in terms of the usefulness of this functionality. Cameras only understand a scene in two dimensions. The shallower the camera angle of view, the more occlusion will happen and it will impact the likelihood of the heat map correctly representing activity for an area. The steeper the angle the more effective the heat maps are at detecting a specific motion.