Carlotta used to go elsewhere if she couldn't get to a litter tray (aka if it wasn't pristine, if Mieze was using it, if Rosa or a shut door was in the way of her favourite one... The list goes on), so I feel your pain there!
The best I could do to combat it was to make sure her favourite litter tray was always very clean and always accessible. Also she tended to go for things that belonged to guests, Rosa or my man so we made sure none of these things were left lying out where possible. It didn't always work as Mieze sometimes used that one too, but she didn't have any "accidents" in her last six or so months so we must have been doing something right.
Hope you manage to get to the bottom of this!
To get rid of the smell and stop her claiming her new place as a toilet, I'd put apple cider vinegar mixed with water and lemon onto the object/carpet/place and scrub it in. After it dried I'd put baking soda down all around the area (and a bit more to be on the safe side!) and leave it for at least an hour before hoovering off. This seemed to stop her going in the same place over and over which she would do otherwise.
It's strange now we don't have Carlotta. We just have one tray, we can get our living room rug back, and the tray isn't easy to get to (there's a fence around it to stop Rosa snacking) but Mieze still uses it no matter what.