There are all sorts of people employed by the security services. There are the so-called professionals at the top, with teams of specialists reporting to them. In this day and age, there is obviously a strong emphasis on technology: communications interception, cyber crime and the like.
But there is also a need for 'foot soldiers'. These are the people who travel around; meeting, talking, observing, recruiting and, of course, disposing of the opposition when it is necessary. They do not have formal identification and, more often than not, they are pursuing another career. That cover, plus the ability to travel unhindered, are key to the contribution they make. They may be businesspeople, academics, writers, ex military personnel or even semi-retired. That last one is near perfect. He sits on a the board of a few charities, occasionally has to travel up to London for a meeting, but otherwise is all his neighbours believe him to be. Most of them are not on any official government payroll and if they are paid, it is often in cash; available from a suitably anonymous office.
Despite the cover and/or the training, these are all human beings; with weaknesses, idiosyncracies and occasional dark secrets. In short, they make mistakes or create indiscretions. If they are important enough, they are rescued in some way; if not, they are abandoned or denied in a biblical manner. Here are just two of many examples I have personally experienced.
On one occasion, I was taking a fairly well-known person to lunch at a famous London restaurant. My reason was straightforward business. However, a few days before the meeting I was informed that this person had fairly strong links to the security services. More importantly, I was advised not to go into the gentlemen's washroom at the same time as him! I have never figured out whether this advice was imparted to save me embarrassment or pre-emptive, to keep him out of trouble.
The other was an elderly Belgian gentleman (not Poirot!) who still did a little work in his spare time. I think that he worked for the French, but cannot be certain. Anyway, on one of his fact-finding missions he discovered an address where a member of the UK security services resided from time to time. This person was known to him socially, but not through any 'spooks' network. At Christmas, they would exchange cards; sent to their home addresses. The mistake arose when the elderly Belgian gentleman sent his annual christmas card to the other address! Age catches up with all of us.