A temperature gun is a device that, when a button is depressed, a laser appears. Where the laser is pointed, a temperature reading will be taken. This will appear as a digital reading on the screen.
A temperature gun can be used to detect significant changes and fluctuations in temperature. It can also be used to identify cold and hot spots but use common sense when doing so. For example, if used next to a window or in a gap in the door the temperature reading is naturally going to be lower than room temperature and this would not be classed as unexplained.
Safety Warning: These guns shoot a laser beam and therefore should be used sensibly. Do not point the beam at a person, particularly near the eyes and face!