Dog Pees Outside But Poops Inside?

I adopted a dog a couple of weeks ago. He's 3.5 years old. He had several homes before he got to mine. When I first had him at home he went to the bathroom outside and did great. This past week he's had some accidents.

The reason I'm writing is because this morning I took him out and he peed, then came to me like he was finished. I reminded him to "go potty" and waited a few minutes. He just stood by me the entire time. Then he started to walk in the house. So I thought he was finished and we went in. I went to the bathroom (for one minute) and by the time I was done he had already pooped in the house. Literally, he did this immediately after coming in the house. There were no distractions outside, I wasn't rushing him, he has no medical issues, we were in the place where he's gone before, and it was a 'normal' poop. I also take him out regularly and we have daily walks. None of the reasons I've read about for dogs pooping in the house fit my situation. It's like he returned to the house for this purpose. It's not the first time this has happened. And, his foster mom told me he would hide and go inside and she finally got him to go outside and threw a party every time he did. He was great the first week - no accidents at all. I don't think this was an accident, but I don't think it was about revenge or anything like that. I just don't know how to stop it from happening.