My indoor, neutered, male cat, probably age 10-12 (shelter adoption so unsure), sometimes suddenly starts meowing very loudly and plaintively while wandering around the house. At times when I've seen it start, he's previously been sleeping or otherwise relaxed. It happens most often in the hour or so to either side of when I go to bed at night. I've had him for six years and this behavior began in the last year or so. My vet has found no clinical explanation; she said that it can be a sign of dementia in much older cats but doesn't think that's the case here.
I haven't made any environmental changes and his behavior is otherwise normal. Because his pupils stay dilated all the time and I read pupil dilation and excessive vocalization can indicate hypertension, we checked his blood pressure. While three readings (in one visit) were a little inconsistent, my vet did not consider it excessive and it was a little lower in another exam a few months later. An ophthalmologist we consulted (because of the pupil dilations) found no vision problems and thinks the dilations are age-related.
I've had one other cat who did this loud meowing starting at around age 17, and three other cats who never did. Is this just normal behavioral variation, or is there something else I should be looking for?