Source for that? There is no way of knowing how regularly it's been performed.
But we do know of one because it was a high-profile case of espionage. It’s a super interesting story.
Yes, people can claim stuff that they know and nobody else can prove. Not every idea requires a double blind study with a large sample size. Sometimes you take things and assign probability values. “This guy sounded like he knew what he was talking about”
Sometimes that’s all you ever get. You should be thankful you got a peek behind the curtain at how the real world works.
It’s wild.