The first thing to expect is a thorough cleaning & review by the store management, Franchise holder, Whataburger Corporate and local health department. That being said and in the absence of other reliable information, a single mouse is not a definitive sign of an infestation. Such vermin do come via from multiple entries, ranging from restaurant supplies to open doors.
That is why they call it “what-a-burger”, you never know what is in it.