Honestly guys, if you've ever worked in fast food, this is not that unheard of.
It usually gets caught before the customer gets it though.
I work in the fast food industry. No mice to be found, and I've worked there a few years now. Have the odd fly bobbing about the store during the warm months, but that is about it.
Personally, I think this guy is full of ****. If there is a mouse, there is going to be mouse droppings, which the Health Department would have found evidence of.