I had it one time - couldnt work out where is was coming until I saw the culprit one day - followed it back - into the next street - and had a word with its owner, to be fair he said he wasnt aware the dog was doing it, I suggested him having more control over his dog but it didnt happen again.
unfortunately you are in the place where the dog thinks that is its new toilet so the habit / routine will have to be broken somehow