behind someone's back

idiomatic

(idiomatic) Without somebody's knowledge; secretly.

Japanese Meaning

こっそりと / 密かに / 人に知られずに

They changed the schedule behind someone's back, leaving several team members unprepared for the meeting.

彼らは数人のチームメンバーに知らせないままスケジュールを変更したため、何人かは会議に準備ができていませんでした。

Sense（1）

(idiomatic) Without somebody's knowledge; secretly.

The employees talked about their boss behind his back.