Sudden Wanderlust in 12 y/o Fixed Male?

So here's the scoop on what's been going on with my cat this week.

He's 12 years old and fixed. We let him outside Monday night. Though he tends to spend his evenings outside, there was a really bad windstorm that night, and he doesn't like going outside if it's really cold or windy. BUT he was unusually hellbent on going outside, so we let him out. In the morning (Tuesday), he didn't come back home as he normally does. He did go to our neighbours' (they love him, and have a kitty of their own with whom our cat is friendly) early in the morning, but they let him back outside when they left for work. We didn't see him all of Tuesday or Wednesday. It wasn't until Thursday afternoon that he made an appearance at our neighbours' again. They let him inside, and called us to say that he was there so we could get him. We had told them that he hadn't been around, so they were on the lookout.

They did say that he was acting strangely though. Not lethargic or aggressive, but "not himself" nonetheless. Normally he's very happy to go visit them, but it's like he needed to be coaxed into their home. When we brought him home, he immediately wanted to be let outside again. Regardless, we kept him in for a few hours, he had a nap, and around 5 pm he was making so much noise wanting to be let outside, so we let him out, and again he disappeared. It's Friday night, and we haven't seen him since Thursday afternoon.

Does anyone have any idea as to what's up with him? He hasn't been neglected, he's well fed. He's extremely bonded with us and with our other kitty. Like I said, he's an outdoor cat, but he's never gone off this long without "checking in" so to speak. You'd think if he wanted to spend time outside, he'd at least be coming home to eat. But nope. We just can't figure out what would be keeping him from wanting to come home, or even to our neighbours. He's not a huge eater, so I don't feel that other people giving him food would entice him to stay with them. He'd just take their offerings and come back to us. Our neighbours give him treats and let him stay with them for whole days, but he comes back to us when he's ready. But it's like his mind is preoccupied with something else right now.

The only other time he wandered off for an extended period of time was about 10 years ago when he dislocated his leg during one of his outings. So his injury kept him from coming home for a few days. But when he was home yesterday, he seemed fine. Tail erect, cold nose. Do cats ever get moody like this and just go on "vacations" sometimes? It's very unlike him. Could this behavioural change be a symptom of some illness? 

I'd love some advice! We're really worried.  

Has your kitty come back yet?  If it were my cat, I'd keep him in until I figured out what was going on.  When's the last time he saw a vet?  Maybe he's ill.  

He came back this afternoon while I was gone, apparently. Ran up to the neighbour when he came home, and then came inside our house. My mother said he acted completely normal, nuzzled around, sat on the couch for a bit, had some water, but then wanted back out. Always goes in the same direction out back. She said he was acting more like himself than the other day. He doesn't seem to be in any pain or distress, and hasn't lost any weight. He's collared, so I can't imagine someone thinking he's a stray and adopting him. It's just weird that he doesn't want to stay home for any extended period of time. 

My past and present cats go outside. If one shows some change in behavior, I wait and observe for one or two days depending on how drastic the change is and then I bring him/her to the vet (if the unusual behavior continues). In most cases, the vet finds something or if not which is very, very rare, he will give a vitamin boost injection/prescription. My kitty is back to normal after that.  It is better for your boy and your peace of mind to visit your veterinarian .   

He came back last weekend, acting like his old self. Ate, played, nuzzled, slept, stayed inside. But he wandered off again on Monday! What we've noticed is that his wanderings happen to coincide with my 4 year-old nephew's visits (he came over on Sunday night). My nephew is extremely active and prone to yelling, and often stays with us for several days. My cat's never been aggressive towards him, but does anyone think he wanders off to get away from my nephew, or that he's jealous of him? Maybe my kitty's recently discovered a secluded place he can stay when the little one is over, and figures, "Oh sweet, now I can catch a break from the noise," so he ventures off?

It sounds like that could be what is going on!  Our 2 (litter mates) were 5 years old (and with us since about 10-11 weeks of age) when our oldest child was born.  They were used to a quiet, predictable house.  Now I have 2 children who are almost 5 and 2 1/2.  The girls are not fans of the kids; but especially my noisy, active little boy!  We are very firm on boundaries with the cats and they have places to escape from him (they are indoor only); but it's still a work in progress to teach him how to behave around cats; how to be quiet, etc.  I'm currently watching sale prices on cat trees to see if I can add another one so the girls have another high perch to escape to.  Right now I have 1 tall cat tree that they share.  It's a favorite spot and I've seen them BOTH on top at once!