Obviously, I don't know the answer but I don't think it's too far-fetched to imagine that an agent of one government (e.g. a female KGB agent working in the U.S.) might attempt to seduce someone in a privileged position (in other words, "with access to specific desired information") in another country's government (e.g. a male physicist working at LLNL).
And, immediately after having written the above, I suddenly recalled a few portions of the SF86[0] and, based upon their existence, I'm inclined to believe it happens much more than what I would initially have expected.
Clearly I work on the wrong systems. :(