An unbeknownst situation is one that someone isn't aware of, often because they weren't told or didn't see it.
You might hear "unbeknownst" used when someone wants to explain something surprising or unexpected that happened without their knowledge. For example, if your friend got a promotion at work but nobody told you about it until the day it was announced, it could be said that she had no idea her job was being changed unbeknownst to her.
(followed by to) without the knowledge of
Example: He went home early, unbeknownst to his mother.
