Shit used to happen with my wife all the time while we were in Phoenix, usually in Walmart. What she eventually learned is that if I have a lot on my mind, I tend to fix my gaze while I'm thinking. In a store or pretty much anywhere else, this can appear as if I'm staring and it caused a number of issues between us until one day she thought I was staring at a woman, but this time it was someone she knew I wouldn't have any interest in whatsoever. She finally asked "were you staring at that woman???" to which I answered "what woman???" and it all clicked in that moment. From that point on she would joke with people that I could zone out and it would look like I was either staring intensely at them or appear as if I was paying attention to what they were saying to me but the whole time a million thoughts and scenarios have just run through my head and I didn't see them or hear a single word they said.