It is not simply a question of keeping details offline, either, but the opposite: individuals or identities without deep, broad online presences are precisely those likely to raise suspicion.

“The challenge of having a credible digital footprint is significant,” Mr Inkster said. Fake Twitter or Facebook accounts alone do not make the grade. “Wherever we go in today’s world we leave a digital footprint “” a digital exhaust.” This too “” a long, false trail of location services on a mobile phone that adds up with an individual’s fake back-story, for example “” needs to be taken into account.

The spy who liked me: Britain’s changing secret service