Sometimes I think it might be a psychological thing.... A few weeks ago my parents went back to the UK after staying with us for 6 weeks and every day after that, Piper went into their empty bedroom and did a poo! I know this because I discovered him doing it after 5 days, and found 5 massive poos!
Other times, Piper goes indoors if it's raining outside. He doesn't like to poo in the rain!!!
Or it could be if Harley has some discomfort going up or down stairs after his operation, he may not be able to get to his usual spot outside?