"Dear [guest], I just looked over and see there is a cat in the window. I am so sorry! I don't know how a cat could have got in. How awful for you! I will be over right away to remove it."
Just kidding. That's just what sprang to mind. You do need to address it, but with something businesslike.
Tell the guest you have seen there is a cat - because it is sitting in the window - and ask how many animals she has with her so you can adjust through the resolution centre accordingly.
It's annoying, isn't it.