I am curious about a comment made in evidence at the London Bridge terror attacks inquest by the lawyer representing several of the victims' families:
There were "opportunities galore" to identify that the London Bridge extremists were plotting an attack, an inquest has heard.
Mr Patterson [said] that there was one occasion in May when all three men were at the gym "in the dead of night" and were speaking together in the street, but employed a "classic anti-surveillance technique" of leaving a telephone on the ground while they walked away and talked.
(Source1, emphasis mine)
From the transcript of the day's evidence (PDF):
Q. Well, they wouldn’t detach - - they wouldn’t stop thinking . They would be thinking to themselves: goodness me, that’s interesting , I wonder what’s going on, wouldn’t they?
A. A surveillance officer records all movements of the individual , sir .
Q. The person to whom he reports it back would be thinking goodness me, that’s interesting ?
A. Potentially , sir , yes.
Q. Yes, especially when the telephone is placed on the ground before the walk and talk begins, a classic anti - surveillance technique, isn’t it ?
A. Yes, sir
(LBI, Day 18, page 7 (PDF). Q - Mr Patterson, A - Acting Det Ch Insp Wayne Jolley. Emphasis mine.)
Some further context is given in the question and answer immediately following this:
Q. And again and again in these sorts of jihadist plots we see that they go to the centre of a boating lake or the centre of a park or an area of open land and walk and talk so they can distance themselves from any suspected microphones or surveillance techniques; that’s right , isn’t it ?
A. There are many different methods, sir
I am curious as to how leaving a telephone on the ground in the street is considered an "classic anti-surveillance technique". What surveillance is this countering?
A phone tap - intercepting telephone communications - would not catch people talking in person. I don't believe that it would defeat correlation of locations either, as the individuals were still present at the same location and (from the description) presumably observed together.
What surveillance does leaving a telephone on the ground while walking away counter?
1: The transcript of the day's evidence was not available at time of writing