Based on body heat, every object in the universe emits infrared energy and that emitted heat or infrared energy is taken as its signature of heat. This emitted energy increases based on the body heat of an object.
A thermal imager often called Infrared Camera is basically a heat vision sensor that uses thermal imaging technology which can detect very little differences in the temperature without any ambient light even in extreme darkness.
A thermal image camera can detect each individual object in the scene and create different thermal images if there are any temperature abnormalities due to leakage in the objects or living beings hidden in the scene. These heat sensitive cameras are used to figure out the temperature differences and heat leaks in buildings and electronics. There are thermal imaging security cameras used in home and industries to ensure the safety. They are also used in army and hunting.