defecation just outside of litter box

Question
Hello,

My girlfriend and I have two cats, about 2 years old, who are brothers from the same batch.  Once in a great while (once every month or so) for the past 6 months or so, one of them (unknown to us) is defecating on the carpet just outside of the litter boxes.  Sometimes it is just a little bit, but tonight... it wasn't.

We have 3 litter boxes (all right next to each other) for the 2 cats, who again are brothers from the same batch, and it is not all the time that this happens.  Because of this, we do not believe (hope) it is a territorial issue, or that we could easily issolate the two cats to find out which one it is.

Also, we do not believe it is a cleanliness issue. We use Tidy Cat Clumping litter, and the boxes are cleaned out every couple of days.  My girlfriend said that once, one of the cats defecated on the carpet just outside the litter box RIGHT AFTER she cleaned the boxes.   

How could we find out which one it is, and what could be the reason(s)?  

Thanks for any advice you can give,
Jake (iamme7680@hotmail.com)

Answer
Going outside the litter box is RARELY a territorial issue with the one defecating outside the box. It's usually one of the other cats preventing that cat from using the box.

here are other reasons:

your litter boxes need to spread out. They are too close. NO litterboxes next to each other. Do you like using a public restroom all the time?

you are not scooping enough(do you flush your toilet every time you use it). Twice per day is minimal for cats.

the cats are defecating outside the box because you guys are doing incorrect litter habits for them. Cats are very fastidious and WANT clean. They hate dirty.

change your litter habits, you will have happier cats