Same thing for me, I kinda hate it when some people snarkily tell me "Hey you still with us?" (I tend to phase out and go in my bubble), I like being left in peace, in my world like djs7p said. I believe that if I have nothing to contribute or anything, I just won't say anything. So, most of the time, everyone around him whether in social events or family reunions is talking and talking, and I'm there in presence, but really, I'm far gone, I tend to look at a spot somewhere, and then fixate that spot for a long time and everything around me just kind of disappears or melts away, it's pretty soothing.
I tend to do that because I hate those kinds of situations, so I just tune out. I had that two days ago with my SO in town, she knows I hate going there because of the crowds, the people, I love being with her when it's just the two of us, but when it's her, other friends, they start talking, and I kind of "lose" her in a way (if that makes any sense), then the discussion is not interesting to me, and I don't say anything. Or I snap out of it for a second to acknowledge something that's been said to me, I have this delay, then answer and go back to my little bubble.
Hell, for a while, I tend to give monosyllabic answers when spoken to, like "Yes", "No"