Bear in mind that ball pythons don't have great eyesight and while he hadn't seen the feeder he had most definitely smelled it. Your hand is warm and moves, and he's smelling mouse and his heat pits are telling him there is warmth and movement nearby, so he will instinctively strike at your hand thinking it is the mouse.
Snakes have an incredibly powerful sense of smell. I can be warming rodents in a sealed cooler full of hot water downstairs in my kitchen, and upstairs in the snake room all of my boas and larger pythons will be up in the front of their enclosures waiting for a rodent to magically appear.