Mine used to do it too. We'd go to dinner and sit at the bar. He sit there at my side staring into space. He'd ask about my day, but just stare straight ahead, nod with no emotion. There was no connection. It was really sad. I always wondered what he was thinking and why he was so "checked out."
I remember it I had been around it for so long, it was such a relief when I hung out with someone one day and we had a two-way conversation with smiling and sharing ideas. I had been in the situation for so long, I felt like something was wrong with me.