We have 2 cats and they're pretty well behaved, toilet-wise. They are allowed out during the day, but we keep them in overnight as they're black cats so would be hard to see if any cars go past.

For the past few months or so though when we've come down in the morning there will be a pungent, sloppy pile of cat poop right in front of the catflap. Their litter trays are literally about 30cm away from the catflap, yet the poop will be there on the floor. it happens so often that we've started moving the litter tray over to the catflap overnight. So now we get the poop in the littertray, but the cat never buries this one - they clear a space in the tray and drop it there, not covering it up at all.

This means our house stinks every morning, which is not great.

I've been looking into why they might be doing this, but can't find any reason. Possibly they're worried that a neighbourhood cat is going to come in overnight or something so are marking the area as a deterrant? But cat's would usually spray for that, not poop. Plus the catflap is chipped and locked overnight, so no cat would ever come in. They're happy cats, and show no signs of illness or distress, so we're a bit at a loss of why they're doing this.

Why are we getting these overnight poops, and how can we stop it?