Cat going outside litter box every 3rd time.

Help! We have 2 10 month old kittens that share 2 litter boxes spaced 2 feet apart in our laundry room. They are litter mates and spayed/neutered. We never had a problem we went on vacation (we had a daily cat sitter), 2 days before we returned home our female started pooping about 1 foot from the litter box once a day (time varies). This has become habitual in the past 6 weeks, always in the laundry room (although not always in the same spot, never more than once a day). We thought at first she was stressed that we were gone but it can't be that since it is still going on. No changes in his time to litter box, litter (unvented) or food.

She is healthy and has no other behavior problems. We have tried everything to clean the linoleum save replacing it (bleach, baking soda, oxy clean, store cleaners, etc.). She will even poop on baking soda residue if I don't get a chance to mop it up.

We have tried retraining her in a different room for over a week without the other kitten. If she is denied access to the laundry room she goes in the box for days, no problems. She goes in the box for all #1. She poops approximately 3 times a day and the other times are always in the box.
We have determined that approximately every third poop is on the floor but no matter what we do we can't get her to stop. We don't think she is afraid of the box or her litter. We don't think it is her brother interfering. We clean the box daily (or more) and that does not seem to be the issue, it seems to be a habit. Any ideas on how to stop this? We are so sick of cleaning poop off the floor.

Just a few more things...
The litter is unscented. The litter boxes are domes and the cats share them, they don't seem to prefer one over the other.. We added a 3rd litter box for awhile even moving it every day to where she (Penn) was pooping and it didn't make a difference, she pooped right next to it. When we moved the litter box to her "safe room" (kitty jail as my husband calls it) for retraining she had no issues with the fact that we moved it to a different floor of the house.
I really don't want to make my laundry room a no cat area as it is out of the way so they can have lots of privacy and replacing the linoleum is not in the cards.
Both Pike and Penn are very social and always together, we've never had any behavior problems with them.

It's odd that a 3rd box didn't do the trick. Some ideas jumped out at me as I was reading your post:

1) When was her last vet visit? I generally advise a vet visit if they haven't had one in a while when there are drastic behavioral changes.

2) Have you tried spacing the litter box far apart (in a completely different area, not just across the room)? Some cats treat two boxes in the same area as one big box, and they don't like sharing. The fact that she is fine when it was moved to "kitty jail" reinforces this. Could it be a change you can make permanent?

3) Have you tried an uncovered box for her?

4) What about trying a different litter?

If you plan to make any changes, do them one at a time so you can tell what works and what doesn't. I would go with the different location first, as I strongly think it plays a part.

Thanks for the suggestions. The 3rd litter box is uncovered. She doesn't seem to care either way.
We have the "kitty jail" litter box still upstairs and that hasn't seemed to stop her from going on the floor in the laundry room. Locking her in "kitty jail" worked because we cut off her access to the laundry room.

Ultimately we would like to move it back downstairs, but we will keep it there if it works!

Next step is vet (she was last there in Nov.) and then different litter, it is strange to me that the litter would be ok most of the time, but then not once a day?

I just noticed that you say she poops 3 times a day? That seems excessive. Or did you mean 3 poops, one poop per time? I ask because 3 times a day can get crowded very, very fast, and it would seem like if that was the case, one or two scoopings per day may not be enough. Perhaps after 2 poops she's thinking it's too crowded. 

Any idea what could have changed in her environment when you were gone? What does the cat sitter think?

Have you cleaned the floor with a good enzymatic cleaner? Does there seem to be any connection between the washer or dryer running and her pooping on the floor? Have you tried adding a fourth box with an entirely different kind of litter, e.g., clumping vs non- clumping? A totally different type of box?

Cats can develop certain preferences over time. We had a similar problem with one of the shelter cats for months and tried all sorts of combinations. It became clear that he'd decided at about 6 months of age that he needed two boxes all to himself, one for #1 and another for #2. It wasn't until he was given a supersized covered box for #2 that he stopped pooping on the floor. There were also two roommates who required 5 boxes between them: one each for peeing or pooping, plus an extra one for those few times when both their boxes had been used and not scooped before they had to go again.

She poops 3 times in 24 hours, she will poop on the floor even when all the boxes are clean. We frequently work remotely so we have tried cleaning the litter box after each use (also did not work).

We run our washer/dryer one day a week and haven't noticed a connection.

The only thing I can think of environment change is perhaps her brother (Pike) scared/startled her while we were gone or she was mad we were not home (it was their first time where we were gone more than 48 hours). The cat sitter also couldn't figure out why. We try to keep them apart when it is close to litter box time and usually they leave each other alone.

I will look into enzymatic cleaners, I'm not sure if any of the ones we purchase qualify (there have been a bunch!)

Thanks!

Here are some suggestions for litter box set up and placement

http://www.thecatsite.com/a/litterbox-problems-in-cats-the-ultimate-guide

 My Angel has the same problem, but only with pooping  At first I thought it was stress related due to some repair work being done in the house  but after everything calmed down  it still happened. We took her to the vet and  they were also stumped.  She was given a clean bill of health. When cleaning up after her "accidents" I noticed  a few things. She has a lot of hair in her stool and the majority of the time it was very hard.  I started putting Petromalt on her paw. She is very picky and that is the only thing I could get her to ingest to  help her get the extra "lubrication"  into her system.  It helped  soften her stool and  She  resumed using her litter box.   When I spoke to the vet about my conclusion she said that if Angel found it painful to go to the bathroom( poop)  she could easily associate  the litter box with a bad experience and decide to poop outside the box.  Although it still happens occasionally I find it is because  i haven't given her the Petromalt. hope this helps