Restroom Misbehaviour

Hello! I have a behaviour question, maybe you can help me figure out what is happening. My 7 month male rough collie is a very smart dog and behaves very well, except for one thing which is happening lately. As as puppy, he soon learned to pee and poo in the pads so before and after he could walk at the street everything was perfect. Then we moved from a flat to a house with a garden and he was even more happy, as he could run and play at the garden all day, although we keep walking outside every day (we now live at the countryside and we do a long walk around 2 hours). Lately he suddenly found a spot to pee on our carpet at night (during the day he always does it at the garden or outside when we are walking), he doesn't do it every night but often enough to completely ruin the carpet. He perfectly knows he should do it in the pads (we did some trips with him and he never missed them) but at home he is always doing it in that same spot. Why is this happening? Any clues? We finally had to lock him in the kitchen at nights, and then he is back on the pads (not using them every night, but when he needs them he uses them). I would prefer not to lock him out of the living room, there's much more room for him and also that makes me lock the cat as well. Yes, there's a cat too, she was home before him so she was there all the time. They don't get along very well but I don't think that's the problem: the dog wants to play but the cat is older and doesn't so she just ignores him; also when the dog was a small puppy was afraid of the cat, but now the dog is 3 times bigger so it's the cat who is scared of the dog. Any hints would be much appreciated, thank you so much!!