Anything, including children, that thought it was food. However, the most likely scenario would be that the mice took it back to their den, ate it there, and died in their den. That is why you don't see any dead mice. There could be lots of mice in the den, so that might be why the bait is still being taken. There could be other animals around that are taking the bait too. But be careful if there are any children around. They may pick it up thinking it's something to eat.