I can only speculate. The older female would, rarely but occasionally, go on a streak for a few days where she would go on the floor overnight even though she always gets left out late. She's a worrier and sometimes forgets to do her business because she worries about her rabbits and her cats. She's made that part of her job. I am not 100% sure it is her The boy is about 20 months and is well house trained to do his business outside. He is marking. This is very recent; just the last week or two. A few months ago my son moved back home with his intact older male dog. The boys are buddies and get along well. Both boys are being neutered on Friday. Perhaps we're not cleaning the area as well as necessary because the girls have their accidents in same area. Tried 3 different cleaners and they don't work. Dogs go back to the same spot. One area is on a worn wood floor. We scrubbed and steamed that area and sealed it. We didn't do the whole floor even though it needs it. Seemed to work for a few days. Maybe, because we now have 4 dogs, this is an attention getter?