Ben Sign is a private investigator.
I had started to say that he "is an agent for MI6" because when we first meet him, that is the case. However, before we get too far along in spending time with Sign, we find we meet up with him around the time he decides it is appropriate for him to move to the next stage of his life.
We discover quite early that Sign has been an operative for 27 years. He was recruited while finishing up his college years as a graduate student at Oxford, the son of academics now deceased with no siblings.
One interesting thing we learn from that recruitment was that "some considered Sign to be too tall to be a spy". His recruiter had told him that "we need grey men who don't stand out" to which Sign "had replied that in order to play the grey man one needed to stand out". I am not quite sure what that reply meant but it was apparently enough to get the recruiter to smile and likely sealed the deal for entry into the Secret Intelligence Service.
Back to our meeting him, we are there when Sign decides it is time to move on. Though he was considered "the smartest man in MI6. One of the smartest men in the country", he had concluded he "no longer wanted to belong to an organization run by buffoons". He chose to instead work for himself as a "private consultancy" consulting on "crime, espionage, mysteries. That's what I do".
He also chose to take on an employee to get someone to help him. He was looking for someone "special", someone from a line of work different that Sign had enjoyed. He found that in Tom Knutsen, a Metropolitan police detective who had had an excellent career there and would likely have still been with them had a very bad man not killed his partner. When Knutsen found that bad man later and put a bullet in his head, it was unfortunate that there was CCTV evidence of it. So Knutsen was needing new employment right when Sign was looking.
Sign may no longer be in the British intelligence system, officially, but with the extensive experience he has, coupled with the now independence (read deniability) that he has, he and Knutsen will find themselves very popular men to call for help.