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