My guess would be that he smelled food but picked up on your heat signature. Its problem with larger boids like burms with strong feeding responses.

Remember these snakes are nocturnal so they really don't rely on eyesight to get around. If I remember right, the eyes of nocturnal animals have a different balance in the number of rods and cones and other receptors than diurnal animals do. They see more in shadows than in definition like we do. In other words, if they see a rat shaped object sitting on the ground that niether gives off heat nor smells like a rat, they will not percieve that object as food.