Ohh... I would be upset too.
I would have to let them know that I hear what they said!
Just think about how bad they'll feel once they know you heard what they said!
(I'm rotten... I know! )

Maybe when you see them ouside (or watch their door and wait for them to come out ) You could say "I couldn't help but overhear your conversation about me the other day, and I just wanted to clear a few things up...then what Karen said... I don't have 11 million cats and I foster...etc."