All snakes have a very sensitive hearing / feeling network of nerve endings on the ventral aspect / belly of their bodies. They can sense minute vibrations and are always on the alert to determine whether they need to go into defensive mode or just retreat. So as we move near their enclosures they can definitively sense our approach. Our footsteps are heard / felt just like that little mouse or rat is sensed. Not withstanding the smell and the little noises the rodents make also. This is partly why it’s sometimes not easy to see them drinking water.