Why is my cat pooping in my family room?

I have two new cats who have been home with me for a week. They have two litter boxes which are scooped daily and which they clearly use. And yet, the boy (four months old) has twice pooped in the corner of my family room. The first time, he went behind the TV, and I cleaned it up and blocked the spot. Today, he went nearby that spot since it was inaccessible. I cleaned that up and have made that spot inaccessible as well. What would make him do this when he clearly knows how to use the litter box and for an entire week has used it all but these two times?