It's not a secret. Hannity was talking about being a member just last week.
Here's the gotcha...
There's a huge difference between having to belong to a union and wanting to belong to a union. My father in law is a retired Delta pilot. He had to belong to the pilot's union. Had no choice. He thought it sucked and didn't support them at all because most of the rank and file thought that the leadership had ulterior motives as many do. Despite that he was a card carrying dues paying member because he had no choice.
I know Hannity falls into that category because he flat said so. I'm sure the others feel the same way...
Since I can proudly say I don't listen to Hannity I stand corrected, it's not a secret.
Now instead of commenting on Wisconsin union members where he is not a member he could bemoan his own union and reveal why he feels he doesn't need them or why he feels they don't represent the best interest of the members or management. He could devote an entire show to the inner workings of this anti-American group he claims he has been forced to join.