at his/her desk, at his/her office
J'ai vu Tom assis à son bureau, en train de lire un livre.
I saw Tom sitting at his desk, reading a book.
Tu es entrée et tu t'es assise à son bureau ?
Did you go in and sit down at his desk?
— À bout de course (1988)
J'ai rencontré Tom à son bureau à Boston.
I met Tom at his office in Boston.
Le capitaine Vidal vous attend à son bureau.
Captain Vidal is waiting for you in his office.
— Le Labyrinthe de Pan (2006)
Tu as laissé ta casquette à son bureau.
You left your cap at his desk.
— In the Mood for Love (2000)
C'est juste votre père assis à son bureau.
It's just your dad sitting at his desk.
He is at his desk.
Tom était assis à son bureau.
Tom was sitting at his desk.
She is not at her desk.