As goldenkint says, sometimes you just look at other people’s carts or stuff (because really, where are you going to look in a grocery?) and think about your own order and what you forgot and what you need, and you don’t realize that you are staring. Once someone asked me if I wanted an item from their cart, for that reason. And I said, “What? Oh, I’m so sorry, was I staring or something? And by the way, are those plates on sale? What aisle?”
So they told me and watched my cart while I ran and got plates.
Usually, though, it’s the person behind you who is staring at you, not the person ahead of you; they are generally busy with unloading and paying and remembering that they forgot the mustard. So it’s unusual that the person in front was staring. Why was he still there?