We have a rescue cat who came to live with us a month ago. For the first 2 weeks he was kept indoors, and used his litter tray when he needed to poo. Then we let him out and he stopped using his tray, and went outside somewhere - hopefully fields near the bottom of our garden although I have been trying to get hold of our neighbour to check he hasn't been going in her garden. (She is deaf and doesn't answer the door).
So far, so good. But then two days ago, after over 2 weeks outside, he has started to poo on our grass. He has done it every day since, although he has also used his tray overnight as well.
Can anyone shed any light onto why this is? And how on earth do we stop him? We had a terrible problem with cat poo from other cats in our previous house and it made the garden unuseable.
If DH finds out that he is doing it then he is likely to send him back to the rescue. 

For now, I am checking regularly and clearing it up asap. I am washing the area of lawn with water as much as I can and have dug out our old infrared cat scrarer thing to deter him from going in that area, but I can't see this as a long term solution. Help!