Cat suddenly defecating outside litter box.

Question
My cat is technically feral, but he's been with us for almost two years now and is essentially a pet. He generally stays outdoors, and comes in to eat and nap during the day. Once in a while however, he prefers to stay indoors through the night. On those days I leave dry food, water and a litterbox out for him. He's pretty good about using the litterbox, though he does use it as a last resort, if no one's up to let him out.
A few nights back he stayed inside, and it was after such a long time, that I forgot to fill the litter box, so he ended up peeing on the carpet which is understandable. But since then, he's pooped on the carpet twice now, in two different rooms and this was during the afternoon/ evening, when everyone in the house was up and around, and his litterbox was full and clean.
I can't understand this sudden change in behaviour, and I'm not really sure what to do about it. I don't think punishing him will help, since he won't know what he's being punished for. For the time being I'm just keeping him away from the carpeted areas of the house, but I really want to make sure it doesn't happen again.
He's eating and drinking normally, and the stool seemed normal, it isn't diarrhea or anything.
I'd appreciate any advice you could give me on why this is happening and how to handle it.

Answer
Hi Sadaf,

The carpet area has his scent in it now. You need to get a good enzymatic cat urine and feces odor cleaner to get rid of the smell. And maybe move a litter box where he was using it. And yes, you are right, punishing won't do anything. He may have had a bit of constipation. And if he did he may have associated the box with the pain he was having. Try using a different type of litter. That may be enough to get him back in the box. You can try using some of this stuff,

http://www.preciouscat.com/product/ultra-litter-attractant/

Works pretty good. If you have a hood on your litter box, remove it. Hopefully the ideas I have given you will help. Good luck and take care.

Ciao, Karen