Oddly enough I am familiar with dealing with this sort of thing as one of my previous cats also had a bowel problem and wherever he sat or lay he leaked liquid poo. He couldn't help it!
When he knew he needed to go, he went to the litter box so it wasn't wilful or deliberate (like this new cat) At least with this cat it is solid and easy to scoop up.
She doesn't do it every day (but it's nearly every day) and I can't figure out a reason for that inconsistency!!
Nothing I have done, (and I have tried a few more things since my original post) has made her use a litter box instead.
I guess I am just have to leave the plastic runner down so it's easy to clean up (and be cautious when I walk near the front door!!
I have no knowledge of her history before she went into rescue, but I do wonder if it was this very problem that caused her to be given up to Cat's Protection.
I can imagine a family with young children wouldn't want such a thing happening AT ALL whereas I can cope with it. I'd prefer not to, but I can