let the cat out of the bag > to let the news/information that you know to be known to others. "except there's no cat to let out". > except there is no or such news to let anyone know ( even if you want to)

The "cat" is the secret and the "bag" is what hides the secret. Mary says that there is in fact no secret which she has to keep yet.

It means there isn't a secret, either to keep or to give away. It isn't true that they have won.

Gordon: You'll probably find out anyway... a little bird just told me that I've won the Restaurant of the Year award! But ssshhh, keep it to yourself. Mary: OK, I won't let the cat out of the bag... "except there's no cat to let out". The awards aren't for another six months - we couldn't have won. So, "I won't let the cat out of the bag" means you will not reveal a secret of some sort, therefore "except there's no cat to let out" would mean there is no secret to reveal. But, I'd avoid talking in figures of speech like this. Used in the right amounts they can enhance a conversation, but if you overuse them they become annoying and/or confusing