Cat sometimes not using litter box

[mzgarden]

I'm stumped. I've had cats for over 55 years but this is new behavior for me.
We have two adult neutered male indoor cats. They are 5 y/o now, we've had them since they were kittens - litter mates. We have two cat boxes that are cleaned every evening (sometimes morning too). No changes to food, brand of litter, location of boxes, feeding schedule, etc. Fecal sample taken to vet - all clear. No other behavior or physical changes. For 3 days now, one time a night, he poos in the house, nowhere near the boxes. Based on trail cameras we know which cat it is and we confirmed that other times of the day he is using one of the cat boxes. I've tried blocking the favored spot, so he picked another nearby spot. Always poo, never pee.

Not sure what to do at this point - anyone have ideas?

[wildhorseluvr]

The first thing I would try is a 3rd cat box, possibly in the new spot he prefers. Maybe clean the boxes twice a day. 

[mzgarden]

wildhorseluvr, thanks. Exactly my though as well. I put a 3rd brand new litter box near his area of choice.

[feather]

I had a female fixed and half declawed that started doing weird stuff like that. She just wanted to emigrate from indoors to outdoors and we stayed friends.

[Use Less]

I would wonder about some other ailment. Maybe time for a visit to the vet. Could the second cat be stalking or blocking access to the boxes?

[mzgarden]

Use Less, thanks. We talked to the vet and are trying a couple other things. Based on the trail camera set up at the normal place for the cat boxes, we see him there. No indication the other cat is bullying him, but it's a good idea to keep watch for. We did put a 3rd box near his new spot and for whatever reason it's been two nights and no poo outside the box (yay). He's not using the new 3rd box so I don't know if it helped or not. Ah well, we'll keep an eye on him and see how things go. Strange kitty.