Hi! This is fairly common in older cats. One of my two did it more than the other. He'd wander off and start yowling a very deep sound in the middle of the night from time to time. I would call him and distract him and he would stop. I have heard a couple theories, one being they do start going a little senile or get disorented when they get older. Nothing to be overly concerned with if it is simply meowing loudly when they are out of sight -- many say they are confused and trying to find you sometimes. If you think it may be more, get him checked with the vet.

It isn't something he is doing wrong or that you should discipline or try to correct -- it is simply confusion as he is older and may get disoriented from time to time.
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Tammy
(Stanley 8/91 - 8/07)
(Pookey 8/91 - 11/10)