Surprising that they would pass all their spies by a single individual. That's a serious operational risk, even if she managed to ferret out some bad apples.
This was being discussed on BBC radio this morning. She was only one of many, but considered the best because she was very adaptable and could pick up many different social engineering tricks easily.
By contrast, another was let go because she carried on relationships with the trainees after they were sacked!
To be fair it doesn't say she handled them all, and furthermore hers is only one file among thousands being made available. Otherwise yes, it would be a terrible risk.
One humint technique is to send her more than just your spies. Some civilian scientist in the RADAR division, some engineer from the jet engine department, a file clerk in working in .mil headquarters... And counter intel, send her out with suspected spies. It wouldn't be hard to convince the other side that she's primarily a counterintel worker. And of course do some counter intel on her, send her some guys with orders to act like German spies, "hey baby, I bet you have a long list of English spies and I have a large pile of money, maybe we could trade" and see what she says. And you can salt the earth by telling her / paying her to tell the Germans a line of BS all day long. "Oh yeah, that crypto clerk who works for Rommel, he's got a huge swiss bank account. Ignore the man behind the curtain at bletchly park, whats going on is all this crypto comms clerk working for Rommel who we're paying off, oh and about ten other guys including of course Hitlers mustache comber..."
This is aside from the rather blindingly obvious solution of keep her busy... if the only guys she talks to are your spies, then when is she meeting these theoretical German spies?
Another problem is you vet 3000 people and its going to be extremely hard to describe them all. "Well, they look english, like bangers n mash, they say "bloody" and "god save the king" a lot, kinda average height..."
From previous releases of files, we know that others also had the job of testing people by chatting to them, plying them with drink and so on - eg Noreen Riols, whose book mentions the Fifi story. But Fifi was more notorious because supposedly her job was to find out if agents talked in their sleep; apparently after 1943 trainee's rooms were bugged instead.
They don't seem to have mentioned that this time around, so maybe that turned out to be just a rumour.
Of course you could say there was a 'need to know' but at a minimum there should have been many 'Fifi's', and preferably ones that did not come from occupied territory.
