I feel this way sometimes but, I notice it more when I'm day dreaming which is why I have to force myself to focus and empty my mind when I have to do something serious or important but, sometimes I can just zone out and be like I'm not there at all. People have even told me that they'd been calling my name and it was as if I didn't hear them. So, they'll shout to get my attention and I'll be like "Huh? What?" Usually my excuse is either day dreaming or simply an "I don't know..."