Feline dementia?

Since Jack's recent serious illness (and before it too) we have noticed some changes in his behaviour which led me to look up the symptoms of old age in cats. Thanks to advances in vet medicine, many domestic cats are living longer now, so they often exhibit symptoms that they would never used to have lived long enough to develop!
Thanks to the joys of google there is plenty of available information. Many of the symptoms of feline dementia I'm glad to say do not seem to affect Jack e.g. loss of appetite, lack of grooming, being irritable, urinating in the house, excessive sleeping (although Jack loves to sleep so that one would be hard to judge!)
But there is one thing that has definitely changed; he now has "increased and excessive vocalisations". Jill was always the talkative one, and had a high, feminine meow - one day when the boys were all jokingly chanting "Mum, Mum, Mum!" I could swear that Jill joined in and meowed her own "Muuuum" as an actual word - we all heard it!
But Jack didn't meow so much, and when he did it was in a gruff little voice. I remember one day on my way out to the car I heard his distinctive voice coming from the garage where he had managed to get himself locked in overnight!
Now however he meows very loudly and plaintively, at random times around the house. It's hard to tell quite why - he is very deaf so maybe he feels disorientated without his former sharp hearing informing him of where we all are. Or maybe he doesn't realise that he is being so loud because of his deafness; the other day he was watching a pigeon through the window and making murderous sounds in his throat that I'm sure he didn't know he was doing out loud. Or maybe he is getting a bit confused? If he finds himself alone in the hall why doesn't he just come in search for us? (He does do that eventually.)
It could of course be pain from his arthritis (I hope not) and the vet mentioned that it might be something to do with his thyroid that she can check out next time we visit.
In the mean time, if I hear my furry boy yowling, I go and get him and give him a cuddle. This usually makes him purr until he gets fed up after a couple of minutes and politely disengages himself from me!