First, only the Carter's twin black cats were called Mu, since my people are slow at figuring out which one is Jake and which one is Elwood. The Mu's make a regular thing of napping underneath the hosta in the front garden, and then they have their routes. If someone notices one of them crouching under the holly thinking robin robin robin here robin, they call out "Muuuu." I come running to that window. As soon as they figured out that I do that, they generalized it. Now they call "Muuuu!" and I know there's some sort of cat out there. Make a note: not one of them has ever gotten in here.
Some of my more esoteric readers may know that chanting Mu is popular in certain Zen traditions. I won't go into the story, it is one of those koans, and they don't make any sense, especially when they are about dogs. Anyway, "mu" is just a Japanese word that means "nothing" or "no", and I don't care how many times people yell it, I haven't noticed anyone getting enlightened around here.