Toilet-trained Dog Doing Toilet Inside House?

I have a 22-month old Jack Russel X Yorkshire terrier. He is not neutered yet and it took a lot of time toilet training him when he was a puppy. However he has now been fully toilet trained since and once in a blue moon has done the toilet indoors. It happened very seldom so we dealt with it at the time and passed it off as an accident. Now though, he has done the toilet indoors for the past 3 days - whilst we are home.

He knows its wrong because as soon as he has done it he runs away and hides. He gets plenty walks, the door is open more often than not and he always comes out back with us so it's not as if he doesn't know where to go. It's almost as if he's doing it out of spite.

Nothing has changed at home, no new people or change of feeding/walking times. The day-to-day routines are the same. He even does it when he's just been out or when the door is open for him to go. When he has done the toilet inside (pee or poo) he has immediately been taken outside and offered the time to do his business outdoors, and indoors has been cleaned and disinfected. Other than these recent days he is usually well toilet-trained.

By AmF