There is, of course, also the rather sinister possibility that former members of SHADO could meet with unfortunate 'accidents' shortly after leaving the organisation. An extreme measure but who knows...?
My guess is that 'most' SHADO operatives would never even dream of talking about SHADO, etc, after they retired or were invalided out of the organisation. I'm sure that anyone identified as being a possible 'security risk' would be taken care of... one way or another. Maybe if UFO had gone to a second series, such a scenario could have been explored. As it is, we'll never know for sure.
By the way, there's an interesting piece of dialogue in 'The Cat With Ten Lives' which suggests that Jean Regan actually knew about her husband's job, whereas in the case of Straker's wife and Paul Foster's girlfriend this clearly is not the case. What did Regan's wife think he was doing when he was away and why had she been 'worried'? Again, we'll never know.