Analysis: How big is the threat from Russian spies?
There may well be Russian spies operating in Britain today, but Katia Zatuliveter - who has won an appeal against deportation - is not one of them and the failure of the case against her is an embarrassing blow to a security service for whom the Russians were once the top target.
In the Cold War, Soviet bloc intelligence services certainly did target Parliament and in some cases managed to recruit MPs and those around them.
Honey traps were also employed - one Conservative MP in the 1960s had pictures of him in the arms of a Russian girl sent to colleagues in Parliament, the media and to his wife when he refused to co-operate with the KGB.
So have those days passed?
Just after the Cold War ended, some of British intelligence's most senior officers made an unusual visit to Moscow. They were greeted at the airport by a KGB officer bearing roses and then went to the Lubyanka, the headquarters of Russian intelligence, for what those present remember as a surreal meeting.
Bread and butter
Hunting for Russian spies had been the bread and butter work of MI5 in the Cold War and the two sides gingerly danced round the question of whether they would go on spying on each other now that war was over.
Both sides came away with the sense that not as much was going to change as some thought. The reality was that despite a brief hiatus in the 1990s, spying has continued.
In a 2007 speech, the head of MI5 Jonathan Evans said that "since the end of the Cold War we have seen no decrease in the numbers of undeclared Russian intelligence officers in the UK - at the Russian Embassy and associated organisations - conducting covert activity in this country".
Some of those intelligence officers are watching Russian dissidents in the UK. Who exactly was behind the killing of former FSB officer Alexander Litvinenko in London by radioactive poisoning remains murky.
Others are seeking to steal secrets, particularly from defence, hi-tech and energy industries. It would be naive to think that this is a one-way business though.
British spies are certainly working in Moscow trying to do the same and some were caught in a rather embarrassing event a few years ago using a "spy rock" to transmit information in a Moscow park.
But whereas spy hunting used to be core business for MI5, it is now more of a sideshow thanks to the overwhelming focus on counter-terrorism in the years since 9/11.
Officials have complained about having to worry about Russian activity here and the extent to which that draws resources away from other areas.
With all this in mind, the security services looked at Katia Zatuliveter and her relationship with a member of the Commons Defence Select Committee - MP Mike Hancock - as well as a Nato official and decided it looked suspicious.
The problem was that there was not the evidence to back up that suspicion.
The Home Office has maintained that it stands by the decision to try to deport the 26-year-old Russian, but the failure to find any evidence of her having been spying raises difficult questions about the decision to press forward.
MI5 appears to have taken a closer interest in the Russian researcher after the detention of Anna Chapman by the FBI in the US. She had lived in the UK and there may have been concerns over similar networks operating here, trying to gain access to people with influence and information which was useful if not highly classified.
But whereas the FBI undertook a long surveillance operation to gather evidence in their case, MI5 in the case of Miss Zatuliveter seems to have carried out an assessment based on what it could see and then tried to confront her perhaps with the hope of either a confession or her fleeing.
That did not work and they may have been surprised when she chose to fight her case.
"If it walks like a duck and if it talks like a duck..." one senior Whitehall official said to me about the case recently. But in this case it was not a duck. And nor was it a Russian spy.