I work in a school with quite a large building. I have my own office, but I spend a lot of time out of it, checking the building, going into classrooms etc. If I make a Bracha on food and then leave my office for a short while, do I have to make another Bracha when I come back in order to continue eating?
If you have in mind that you will eat all over the building, so long as you don't go 72 consecutive minutes without eating, the bracha is still good. If you did not have this in mind, you cannot go out of sight from the room where you started. i.e. the bracha dies and you have to make a new one when you return to the original room.
There might be a catch to do with eating something you didn't expect to eat.
No sources on me right now. Advice is "as is." If you find yourself in Gehinnom, please contact your local service provider.