Penn Gwinn is a former agent with the DIA.
When he worked for the Defense Intelligence Agency, which he did for nearly 15 years, he went by the codename of Penguin, obviously due to his name, and he took the occasional ribbing with a chuckle and a "move on". His service with the DIA took place during some of the heaviest actions of the Vietnam War and he saw far more action than an operative for that department of the Pentagon might normally have. Certainly it was far more than was publicly admitted. The "glory boys" of the CIA were thrilled to throw their weight around. Men like Gwinn and his team did their work more quietly.
But as the recorded adventures begin, those days of action are already several years behind him and he has moved on with his life. Moving on is something he seems to do a fair amount for judging by the different occupations he has during the years depicted in the books, he is not one to stay with the same thing for too long.
The first book has Gwinn as an expatriate former operative living and working as a bar owner in Bangkok, Thailand, with his former DIA colleague and best friend, Starret. They had jointly purchased the establishment when they decided the work they had been doing for too many years was not for them anymore. Since they both enjoyed to drink and both loved the Far East, even after the jungles of Vietnam, they acquired the place and were very happy. The problems told in that book would bring an end to that next phase of his life.
San Francisco would be his next landing spot and his occupation would be that of a private investigator. How he made the transition is never explained nor is how he became as established as he was. With a partner named PZ, a new best friend now that Starret was not around anymore, he would do his share of keyhole peeping and errant spouse tailing but bigger cases would also come around that would throw him back into the dark alleys of his former employment.
After a couple of books, though, the partnership with PZ would end and his life as a Frisco gumshoe would largely fade away. He would still refer to himself as a private investigator but he also would enjoy a mildly lucrative living as a relic hunter in the rain forests of Central America. Again, though, he would find that his old line of work would come back, bringing clients and trouble.