3 year old suddenly started peeing indoors

My dog suddenly started peeing in one specific spot in the house. He's a neutered 3 year old, house trained and has a doggie door about 4 feet from his chosen spot. How can I curb this behavior? I haven't been able to catch him in the act. What's the best way to sanitize the area to hopefully discourage future incidents?

First of all, see the vet to make sure your dog doesn't have a urinary tract infection or another physical issue that is causing him to pee indoors.
   
I think it is unlikely, though, but check just in case.
   
Next, clean the spot he's pissing on with Nature's Miracle or the like. The smell must be completely irradicated. Otherwise he will continue to wet there because it's a siren screaming at him, 'PEE HERE, PEE HERE' and he can't resist.
   
Barring a UTI or the like, I think the real problem is that your dog has become a lazy slob who can't be bothered to go outside to pee.
   
He knows it's wrong because he holds it until he knows he can get away with it, then he's unloading.
   
Since you aren't punishing him because you aren't catching him in the act, there is absolutely no penalty for peeing in the house so he's going for it.
   
Why not? All he has to do is wait for you to turn your back, then release his bladder and go on with his day, confident that you won't correct or punish him and you'll clean up his mess.
   
This stops NOW.
   
If you are cleaning his pee while he is watching you, stop. This reinforces that it's OK to pee indoors because you, the cleanup committee, will hop to it and clean up after him and not yell at him or correct him.
   
Since he is not trustworthy, tether him to a reliable human at all times. Now he can't slink off to his new pee spot. If he squats or leg lifts he can be stopped, corrected and taken outside for a correct potty and praise.
   
If he doesn't like it, TOUGH. He lost his freedom when he became too lazy to go the extra inch and use his doggy door.
   
Take him outside every hour or so, just as if he were a little baby, for a potty break, to try to get him back on track (in essense, remedial potty training).
   
If he is tethered to you and lifts his leg, give him a stern "FIDO NO! NO INDOOR PEE" and drag him outside. Whenever he pees outside, he gets "GOOD FIDO, GOOD FIDO PEES OUTSIDE."
   
I am not a fan of rewarding a dog for a correct potty because then they will leg lift, squirt 1 ounce of pee, snap their heads around for their treat, then repeat, repeat, repeat.
   
It will take an hour for the dog to take a leak and he'll get fat as a pig.
   
If all else fails, diaper him. He only has the diaper off when he is outside but inside, he wears his diaper.
   
If he pees the diaper, leave it on him for a good half hour. The goal is for him to understand the misery of a wet diaper. If you change him the moment he wets he will be comfortable and will continue to happily pee his pants, secure in the knowledge that you will change him immediately.
   
I don't think it will come to that, though. I think this bum just needs remedial potty training and some tethering.
   
Good luck, hope your house is soon dry!