This video demonstrates how the HotSpotterTM indicates local temperature differences, rather than absolute temperature. This is important because it may be that the area under investigation is already above ambient outside temperature. What is needed is to find any points or areas where the temperature is above the surroundings. Then investigate further.
So the HotSpotterTM continually adjusts itself to look for changes. It is easy to demonstrate this for yourself: Point the HotSpotterTM at something already warmer than the surroundings (such as your hand) and then switch on. The HotSpotterTM thinks this is ambient and starts up at the low frequency and green LED. Now move your hand away for a short time and then back again. You will see that the HotSpotterTM has adjusted itself to the new lower surrounding ambient and now detects the Infrared from your hand.
So as you scan back and forth the HotSpotterTM will always be checking for changes in temperature and indicating any areas that are hotter than surroundings.