I wish all cat owners would train their cats to use a litter tray. That way I wouldn't get a handful of something nasty hidden in the soil when I'm weeding and planting. ...and please put a bell around darling kitty's neck to save the bird population from carnage.
At least with a Fox it's obvious and you can use a trowel to scoop and remove.
Cats can sneak up on birds even with a bell. Also whilst not the majority, some cats don't like bells. Cats hearing is much keener than ours and having a constant ringing whenever you move doesn't seem very nice. Still it all depends on the cat
I can imagine your frustration. My grandparents lived in a bungalow with other cats about, and the odd one or two made visits. Its in their nature and the owner can't stop them. A dog yes, but a cat does what it wants and there's only so much an owner can disipline them on. Of course you could always invest in a sprinkler system
I have two cats of my own and I love to garden. My cats poop in ours but I accept it. Although my female cat has a habit of peeing in pots.. o_O
Anyway, see if you can spot the pooper in action to see if it is a cat. As people have said they usually do it in loose soil and not the grass. Wish you luck though ^_^