Thereos keeps no office to speak of. Stuffy and unnatural things.

Instead, located a safe distance from the edge of the Forbidden Forest and in clear sight of Hogwarts castle, is a mossy covered stump. Returning students and faculty have likely passed by it without so much as a second thought countless times, but Thereos has a fondness for this fallen oak and chosen to use this location for his "office". Simply sound the nearby farm bell to alert the centaur that you wish to speak with him and he will appear.

If he feels like it.