Outside cat 'pooping ' in garage!

We have 2 outside cats, 1 who is approx. 12yrs old - female spayed at approx. 1 yr - a stray, so no definate age.
She has been the best cat ever and has always been outside. We have a cat door in the garage door , but never trained her to use it so it is always open.
Over the years we have feed !!!! # of stray cats who have come and gone. Also possums! We always have plenty food down - infact they all get speacial food as MomaKitty has bladders problems.
During all these years "she" has sat on the roof of the car and watch the coming and goings of numerous guests.
About 18mnths ago a black male kitten appeared and much to our "amazement" it was slepping in her bed along side her. Something we never thought we would see.
Any he made a home with us and we had him neutered.
About 3/4months ago one of them has strated 'pooping' in the garage. Of course we don't know which one!
We realize MommaKitty is old and it could be that. Although we clean the mess the smell is awful.
We are very relutant to start with litter boxes.
Any suggestions on how to curve this behaviour.
Thanks

That's very surprising. Cats are drawn to sandy soil by nature to do their business. One of my indoor kitties still lives to get out and pee outside. It's like Christmas to him when it happens.

I would suspect as you've said the older cat. Maybe she doesn't feel safe going out? Still, 12 isn't all that old for a cat. Perhaps it would be smart to take a trip to the vet.

Then again, the new kitty could be the culprit since this hasn't happened before.

I've no idea how to stop it.

I have the same problem. I take care of 2 ferals, I had the litter spay/neutered. Only these two stayed around brother and sister, I have a nice place for them in my shed, containers with blankets, water and food. There is a new boy comes around but he is owned by someone down the street, but I notice that now there will be some poop in the shed, I figured someone is marking their territory. Its not all the time, so when I do see it I pick it up and spray the spots with enzyme cleaner. Good luck.

Are you positive it is one of them? My cats also go outside and the only time I found feces in the wrong place, it was made by an aggressive feral who wandered inside the property. Like aswient, I used an enzyme cleaner. My cats shooed away the intruder for good. No problem ever since.