Translation:Who is in the office?
Yes there is a question in earlier lessons where the answer is
谁在洗手间里 or, 'who is in the bathroom?'
洗手间 is specific, but the 里 is added to make it 'in' the bathroom.
There doesn't seem to be much difference between the statement above and the one I've compared it with here.
I've noticed in other sentences that, whenever the word "in" is mentioned, even though it's obvious from the context that we're talking about a location, e.g., inside a room, that the character "li" is always included as well as "zai", and is expected in the answer. I wonder what makes it different here?