1. happening or existing without the knowledge of someone specified — usually used with to
2. not known or not well-known : unknown

In Context

“… Senate Bill 15, approved unanimously by that House committee Thursday, hopes to help homeowners who find themselves the victim of ‘squatting’—people who illegally move into a home, often unbeknownst to the homeowner.”

Marianne Goodland, The Gazette (Colorado Springs, Colorado), 12 Apr. 2018