You would be Very surprised how often things of this nature, and worse, happen.
Have you ever worked in a factory?
Many of them are not the cleanest of places.
Fortunately food packaging facilities are kept in better condition; But in the winter, rodents will always find a way into warm places.
No amount of poison, traps, and/or predators can prevent this 100% of the time.
Never ever ask yourself how they're able to keep billions of bugs out of massive vats of peanut butter.
What we actually eat can be a scary prospect, but unless you're raising your own animals/fruits/vegetables/grains and making all of your own foods, then chances are you'll never know just how many 'contaminants' you come into inadvertent contact with.
True, mice are a fucking big contaminant...but at least you're able to see that they're there.