As I expect you can see in the picture to the left, there is a substantial hole which rats have used to access the wall cavity and associated staining, known as smear marks. unfortunately, the pipe then passes through the inner wall into the kitchen sink pedestal where a much smaller gap existed. Not deterred by this the rats eat away the pipework to gain access into the kitchen.
Twenty four hours later the homeowner filled the sink, washed the plates, pulled the plug and got wet feet. As you can see, the rats have beautifully replicated the original damage to the pipework. The moral of this little story is to get in a pest controller before the plumber next time.