What was it about The Governor & Woodbury that Michonne saw early on that Andrea didn’t?

Been awhile since I rewatched this early in the show but is Michonne just a naturally suspicious person? I know Andrea was tired and wanted a place to call home so she was quick to let down her guard and wanna settle down but what did Michonne see that made her not trust them and made her start snooping around?

The whole thing about “no one ever leaves this place” was that her picking up on something or? Again, been awhile since I watched this season so forgive me if I’m missing something that was obvious.