Security News > 2020 > September > Matt Blaze on OTP Radio Stations

The idea behind fieldcraft is in the case of "Dead drops" to create a "Disconect" between a Case Officer under Diplomatic cover and an agent who will end up against a wall one morning if the Case Oficer does not do their fieldcraft job right.
Because there is a high probability the case officer is under observation and they have to check for the agents tell frequently their behaviour patterns must cross the place where the tell is thus the security services will also know where a tell might be although they do not know what or where it is.
The case officer clearing the tell is the give away of the what and where, thus patient observation once known to the security officers gives them the agent and thus some thumbs to put screws upon.
Overly simplistically the agent sets their tell, when the case officer sees it they set their tell as a response.
The agent clears their tell as an acknowledgment and when the case officer sees the agents tell is clear they clear their tell to close the prorocol.
News URL
https://www.schneier.com/blog/archives/2020/09/matt-blaze-on-otp-radio-stations.html