I think Cataholic is right. I think you have other backyard visitors making her mark her spot. A box likely wouldn't help. I would get a good cleaner like Nature's Miracle to clean the area of Baby's scent so that she can't even find it. It might be a lot of work at first, but she will get the idea that she isn't leaving a mark. Then maybe introduce a box outside if she still feels like she has nowhere else to go.