Your brain is constantly filtering for what deserves your attention.
Your brain is constantly and unconsciously seeking what’s most important in your environment. It is biased towards what’s new, unusual, important, and/or what’s primed.
If you’re looking for Waldo, it’s good at spotting red and white stripes. Regardless of your state, it’s good at spotting snakes. It reacts to alarms and if the smell in the room suddenly changes.
It is prone to missing obvious things, though. Such as the Gorilla who walks through the basketball drill.