Adult dog soils the carpet at age 7

Question
My dog is 7yrs old and has always been very well behaved. Recently she's been going to the loo on the carpet at the other end of the house (less used rooms). She has 24-hr access to the doggie door and can go outside any time, day or night. She sleeps in the room with us, with the bedroom door open, but she usually goes to the loo on the carpet in the morning. For the rest of the day she goes outside. We brought her to the vet because it's very odd behaviour for her. The vet gave us antibiotics to give her, saying it was a UTI. The antibiotics haven't worked and she's still doing it. What's causing her to do this? Any advice you have is very welcome.

Answer
First I would take her to a different vet and have her checked out again. She is young for it, but she may be suffering from dementia. In the meantime, close off the other end of the house before you go to bed so she can't get to it in the morning. This may do the trick and once she gets back in the habit of going outside you can reduce your vigilance.