House Trained. Then all of a sudden...

I have a little terrier that I've had for several months. He was a street find that had been homeless for weeks at minimum, but must have been a pet because he was house trained, and follows a few commands.

So, I kept him and everything was fine for about six months. I let him out into the yard for his personal business.

Now, for the last couple of weeks, he has started doing his #2 in the house. Never the other. #1 he will do outside immediately, and I give him plenty of time.

He likes me and isn't neglected. This came out of nowhere. Any ideas on what to do to stop this? Leaving him out all of the time is not an option. He gets plenty of time out, as much as he had before this habit started. It's as if he's determined to do it in the house for some reason. He doesn't seem to have any health issues.

He usually does this in a little-used area of the house. The other night, he did it on a piece of my clothing that he took from the laundry hamper. I was not too happy.

-A