noun, noun or participle which takes 'suru'
being in a building (embassy, museum, etc.)
Used for being present inside a specific institution or facility, often in official or formal contexts. The type of building is usually clear from context.
大使は本日、在館しております。
The ambassador is in the embassy today.
在館時間は午前9時から午
The hours of presence in the building are from 9 a.m. to 5 p.m.
noun, noun or participle which takes 'suru'
being enrolled (in a school, etc.)
Rare usage, mostly seen in formal documents or older texts. Modern Japanese more commonly uses 在籍 (ざいせき) for enrollment status.
See also: 在籍
彼は当時、大学に在館していた。
He was enrolled at the university at that time.