Why doesn’t the CIA just come out and tell all their recruits, we trust you and most of you truly believe in the mission and will never compromise but there are always a group of turncoats who will sell secrets or have other affiliations. Due to this we will monitor your credit and you will not know if the persons you are working with are internal investigators and we will have randomized investigations as matter of course up to the director.

In other words don’t ignore the possibility but work with that limitation.

Maybe they do this and people still do it, if so they’re doing a poor job of it.