OK I will try to answer all these questions LOL
The cat's litterbox is in our bathroom, which is in his dog free area. He has our bedroom and the bathroom during the day, and the run of the house at night when the dogs are asleep in their area. The one thing that makes me think his messing in the house is not a medical issue is that he poops outside the box too. I clean his box whenever I see it is dirty. He seems to have a fixation with it. Before he started going elsewhere, he would dig and dig and dig at it. He'd go back and dig some more after he'd gone, and steal socks, underwear or any other small items and dig those into the litter too. It's jumbo sized and about 4" deep in the gravel kind of litter. I clean it with Nature's Miracle when I change the litter out. We've washed everything that he's peed on (my husband's clothes), but he will pee on any of his laundry he can find. His laundry is only a few feet from the box, the other side of the bathroom door. He is neutered.
His scratching post is in our bedroom, it is a carpet one. He used to be very good about using it but not so much anymore.
He can't go outside because he has no canine teeth and is longhaired. He had abscesses before we got him, and hadhis teeth removed, another thing the shelter failed to mention. We wanted him as an indoor mouser as well as a pet. Our dogs are large, ranging from 50-90lbs. They like cats, but one in particular loves cats. She wants to groom them and play with them, and found out the hard way this cat did not feel the same way when he scratched her on her rattlesnake-bitten face
He came from a military family that had to move overseas, via the shelter. The shelter was not the kind with individual pens, all the cats had the run of a converted house (30+ cats). He'd been there a year. Nearly all of the cats there were very overweight (20lbs+), but he was fairly skinny. There were some cats (the really really fat ones) that guarded the food bowls. He has issues with covering his food bowl with stolen items too.
He's generally friendly but he does have an attitude. If you brush past him or nudge him he will turn and hiss and spit. We have a barn cat who we moved inside this evening as she is pregnant. Completely off topic, can a domestic cat be bred by a bobcat? We have the only domestic cats on nearly 300,000 acres. We'll get her spayed as soon as she has her kittens. He likes her, and before we brought her inside he would sit on the doorstep with her.
I'm thinking jjthink might have hit the nail on the head, with him being on his best behaviour to begin with and now feeling comfortable enough to be a pest.