Ok, an ambush predator does not always just sit there and literally wait for something to be within reach...if something is in close enough proximity and they can get it, they will. When you're feeding in a tub, that rat is always in close proximity. In the wild, the snake follows the scent of the food source because if there is a fresh scent, there's a good chance that the food source will be coming by again. They are not actively seeking prey as much as they are seeking a place where it is likely prey will pass by.