What mental disorder does this fall into?
This is totally bouncing off of me. How can a person, in a public space, surrounded at all times by other people, just forget they exist for any amount of time, for any reason? They're fucking everywhere. They're breathing, they're talking, their cart wheels are squeaking, the footsteps from their rubber-soled shoes are echoing off the hard tile floors, how do your senses just stop registering any of that?
It's not just me. If I was literally the only other person in the store, sure, I could understand that, they thought they were alone, they weren't expecting to encounter anyone else.
How the fuck do you just stop being aware of an entire seething mass of other humans flowing around you?