Where do secret service agents apply to investigate foreign threats?

My wife worked as a postal worker in Trinidad, she worked at a small PO inside a grocery by a mental hospital and patients would mail letters to heads of state around the world. Often incorrectly addressed (Queen of Russia, Prime Minister of America) often basically jibberish that resembled song lyrics more than threats(finger prints of the mind that change in time etc).

Yet without fail around every six months two agents would walk in and want to talk with her about who sent a letter, she knows for sure she has talked to secret service agents(USA) and their British equivalent. I always wondered exactly how you get on that division? Oh hey Johnson another vaguely dark poem came in from the Bahamas! Pack your bags!