Those of you who have read Alpha to Omega, will know how I used this in the book. As I've previously stated, I have drawn on much factual material in the construction of Alpha to Omega and this is one small example.
The use of the technique was explained to me by a security services officer and then effectively demonstrated. It arose out of necessity, but with mildly amusing consequences. It also showed that even when you are sitting in meetings with people you think you know, you still need to be cautious what you say and how you say it. It happened like this.
Over a period of time, I had been attending meetings at which colleagues not only from the UK, but also France and Germany were present. A couple of days after one of these meetings, one of my UK colleagues said something along the lines of: "your comments about X were apparently raised at a security meeting in Paris yesterday." You may imagine my surprise. Not only that business conversations were being reported back to some official in the French security services, but also because the matter was so apparently mundane.
Not long after, it happened again; this time, relating to a matter my UK colleague had raised in a meeting. This was slightly less surprising, given his status, but still amazing given the subject matter. He said he was not certain who the conduit for this information passing was, but suggested a strategy to 'flush him out'. This basically involved the two of us (by prior mutual agreement) concocting spurious, but reasonable stories and then running them in different gatherings. By noting the 'membership' of the meetings in which the stories were run, it would be possible to identify the conduit back to Paris. Within a couple of weeks, we knew who our man was. It seemed pretty certain that he was debriefed on a fairly regular basis and they had swallowed (or at least been interested in) a most ridiculous, but plausible, story.
What was perhaps the most intriguing, to me, about this whole matter was that a UK security officer was getting feedback from meetings being conducted by the French security services. That is how we (or rather he) discovered our 'mole'. But that of course means UK security services had someone inside French security services who was able to tell them what was being said. Everyone spying on everyone else!
The moral, I suppose, is don't always believe what you hear or read.