Every foreign correspondent learns to spot them - the first or second secretary you run into at the embassy Christmas party, charming but utterly vague when pressed on their line of work. "Bit of politics, bit of economics, liaison, that sort of thing." The Diplomatic Service List gives no clue, and the Foreign Office will not say a word.
What is slightly unusual in this case, intelligence experts said last night, is Mr Hurran's senior rank. As counsellor, he is the third-ranked diplomat in the Prague mission. But he may not have been the only intelligence officer there.
In important posts such as Moscow (and quite possibly Prague), a two- tier system might operate. There could be a "visible" MI6 man - in this case Mr Hurran almost certainly - whose identity was known to the host government, and as such was perfectly placed to deal with "walk-in" defections during the Cold War.
Then there would be the invisibles who never broke cover, whose identity was (theoretically) unknown to the host government. They could be first or second secretaries or even their spouses - as was Janet Chisholm, wife of an MI6 man in Moscow, who was contactperson for the super-spy Oleg Penkovsky before he was caught and executed by the Russians in 1962.
Even the fact that Hurran is gay is less unusual today. Back in the mid- 1980s, Sir Antony Duff, then head of MI5, pressed for homosexuals to be permitted to work for the service. Today being gay is no disqualification for a posting abroad with MI6 or the Foreign Office; indeed, it can be an advantage. For one thing, single people cost less. For another, spouses can be big problems in their own right, especially if obliged to give up work and live in a country they may detest.
The Hurran "outing" is unlikely to have been caused by nostalgia for the old days when Czech and British spooks were on opposite sides. In contrast to the incompetence displayed by the BIS, the modern Czech intelligence service, over Mr Hurran, the Czechs were much admired during the Cold War - "as good as, or better than the Russians," one MI6 veteran recalls.
"Almost certainly many of the old people are still there, you simply can't clean out the Augean stables entirely ...When ideologies change, secret services are like the wiring in the house that's being sold. The new buyer turns the switch, and the lights still work."