Isolative behavior

Question
I have two dogs, the oldest and the one in question is a 9 y.o. neutered male, likely to be a flat-coated retriever mix. Mellow, obedient, lovable. Have had Riley for the past 6 years. I adopted him from a vet tech program adoption at approx. the age of 2.5 y.o.
The second dog is a spayed female flat-coat adopted at age 2 y.o. from flat-coat rescue.She is currently 5 y.o. She is lively, active, likes to groom her brother, wants to be the alpha dog, and Riley usually allows this.
It appears as though Riley has taken to his 'clubhouse' far more frequently than in the past, and appears to be more 'sensitive' to noises. I should mention, that he had been shot at, with 2 or 3 BB pellets remaining under his skin as proof. He has never been fond of loud 'popping' noises such as fireworks, cars backfiring which is clearly understandable. Now he wants to take to the garage or his clubhouse when he hears the oven timer (an electronic beep). But even aside from that, he seems to spend more and more time away from me and the other dog. He appears to be healthy, not in any obvious pain; vet exams reveal no concerns.
Any idea why he would be isolating himself more and more?

Answer
It's not unusual for middle-age to older dogs to become more sound sensative. However, a few things you might ask your vet about which might not show up in a routine examination. Has he had a six-panel thyroid test? If not, I would. Hypothyroid disease is common in older dogs, and can lead to changes in behavior like those you describe. If he hasn't had a recent screening for tick borne diseases, I think I would check that too. And ask your vet about canine cognative disorder (which is similar to alzheimers in humans.) And there is medication which can help if that is the case. Sandy Case MEd CPDT www.positivelycanine.com