You would probably miss the gorilla
A 1999 experiment asked people to count basketball passes, and about half of them failed to see something enormous.
In 1999, the psychologists Daniel Simons and Christopher Chabris filmed two teams passing basketballs and asked viewers to silently count one team's passes. Partway through, a person in a full gorilla suit walked directly through the middle of the action, thumped its chest, and walked off — visible for about nine seconds.
Across their trials, roughly half the viewers never saw it. Not glanced past it — didn't register that it happened at all, and were startled to be shown the replay. Simons and Chabris called this inattentional blindness: attention isn't a floodlight covering everything in view, it's a spotlight, and whatever falls outside its narrow beam might as well not exist, no matter how large or strange.
The unsettling part isn't that we miss things. It's how confident we feel about the things we didn't miss — the sense of having taken in the whole scene is itself an illusion the brain generates after the fact. You weren't watching as much as you think. You were watching for something specific, and everything else was never really in the room.