The fact that she doesn't go crazy when you are in the office, and she's fine with people outside the office, tells me there's something going on between your dog and your husband inside the office. I bet that he is, perhaps unwittingly, rewarding her reactive behavior. It's very common for people to pet and coo at their dogs when they are barking at something, ostensibly to "calm them down"... but the dog interprets this as praise.
Does she obey him generally? How old is she?
My GSD barks at people on the other side of the fence, but a sharp "quiet!" from me and she stops. If I were to say, "It's okay," she would interpret this as praise and encouragement.
My Akbash Dog, on the other hand, has a totally different temperament and thought process. He feels duty-bound to alert, but doesn't really want to bother himself to *do* anything. With him, "it's okay" means "I heard you and I'm taking care of the situation", which is all he seems to want--to know that I've been alerted, so he can go back to sleep.
If I were to reprimand or use a sharp tone to quiet him, it would confuse him. "Why are you yelling at ME? You should be paying attention to the thing I'm barking at!"
I suspect your dog is more like my own GSD. With GSDs, barking can become a sport in and of itself; scaring people is fun!
I think the key is to let your dog know that YOU are in control of the situation and her barking is not necessary or desired. Most GSDs have a strong desire to please their owners, so it helps to let her know when you are NOT pleased with her behavior. Interrupting the barking and giving her an alternative command like "platz" or "go lay down" gives her something else to do than WILL please you.