“Let the cat out of the bag.”
There are two possible origins for this phase. The first refers to the cat-o’-nine-tails whip, which was used as punishment on Royal Navy ships. If a sailor were to rat out another, he’d be letting the “cat out of the bag.”
The second refers to the common scam where they’d be buying a suckling pig, wrapped in a sack, only to find they’ve got an angry cat when they get home.
Either way, it’s about revealing information that someone already didn’t know.