Potty Training Mishaps

My dogs are by no means puppies anymore. They have been trained for quite a long time. But what about that dog who suddenly starts going pooh where he shouldn't? I have just recently discovered that one of my chihuahuas are now thinking that they are allowed to run into the main hall bathroom and do his business whenever that door isn't shut completely. He not only decides to pooh in the wrong place, but he has been doing his business on the rug. Not only that, it's always the most prettiest rug in that room, and when that rug isn't available, he goes on the shower rug. I don't know what's gotten into him that he thinks that he needs to do this. I had been trying to keep the door shut, but yesterday, I found that after everyone had left for work, someone had left open that door, and I caught Shorty-dog coming out of there, just in time, before he left his business on the floor. I know it's him because the other 2 chis I have don't like going in there because that's the bathroom where they get their baths at. Now, the question is, what can I do about this?