I think your first suggestion, that she somehow learned he was under special scrutiny is plausible. I do know at this point her English instruction lessons were being given exclusively to Minsk VIPs who certainly may have let something slip in her presence. Your latter suggestion however is naive at best. From what Ernst told me, opting out of this sort of forced surveillance was not a luxury anyone had. And additionally, from what Ernst told me, Mary had been fired from The Foreign Language Institute in Minsk where she worked during Stalin's reign under the phony pretense that her Russian wasn't adequate to teach the students. Yet later on apparently it was perfectly adequate to teach the Minsk elite! What would she have done had she lost that too?
It does seem very strange that the KGB (as far as we know) never forced her hand and made her meet with Oswald. If they thought his Russian was never very adequate or that he was holding something back, might he have been more loose tongued around a fellow American?
Any thoughts?