The mouse wasn't moving that you could see. Consider that a mouse has a much higher heart rate; if you can pick one up (eg those rare occasions where you are able to rescue prey from the cat) they practically vibrate in your hand.
Also, another poster makes a good point about cats' hearing. Not only do cats hear in higher frequency ranges than we do thanks to their reduced size, they can angle their ears to 'focus' on a particular sound; being a sound engineer by profession, I desperately envy them this ability. I have simulated this using cardioid microphones while wearing headphones, and I would cheerfully sacrifice a good deal of social standing for a pair of working prehensile ears.
Also, another poster makes a good point about cats' hearing. Not only do cats hear in higher frequency ranges than we do thanks to their reduced size, they can angle their ears to 'focus' on a particular sound; being a sound engineer by profession, I desperately envy them this ability. I have simulated this using cardioid microphones while wearing headphones, and I would cheerfully sacrifice a good deal of social standing for a pair of working prehensile ears.