Just imagine the earth and her body heat as a certain shade and hue of red, or green, and loose earth of NOT the same density, ergo, warmth - it will show up lighter.
Actually, that makes sense. The less dense material will heat and cool at a different rate as the surrounding dense material as day and night passes. Add to that an unnatural pattern of loose and compacted soil, e.g. a grave, and you've got yerself a detection system.