The quick answer is yes! While it is sounds like something that could happen in a horror movie this actually does happen. According to Dr. Niamh Quinn, Human-Wildlife Interactions Advisor with the University of California Cooperative Extension, “Rats are excellent swimmers. They can hold their breath and also tread water for several hours”. Prevention can be a challenge if they are coming from the sewer system. Advise your customer to reach out to their local city so they are aware of the situation. Do a thorough inspection to ensure there are no gaps near the plumbing and hopefully once the toilet is back working and has water, they will not return.