What is a field office in the sense of a secret government organization?
Many companies/organizations have a staff of people who work at the main plant or the home office, and also have people who work "in the field," as salesmen, repairmen, undercover agents, etc. If they have a lot of activity in a distant area, it behooves them to staff an office there. It's called, strangely enough, a field office.
I once did "field service" for a machine tool company, but spent some of my time in the main plant.
'Where's Avangi?? (reply) He's in the field. We had offices in the major industrial centers.
Thanks, Avangi!