Schrödinger's Cat's in the Bag
Referring to a secret/rumor you're not sure has gotten around
Person X: Has the secret/rumour gotten around yet?
Person Y: Dunno, Schrödinger's Cat's in the bag
Person X/Y once rumour has gotten around: Guess Schrödinger's Cat is out the bag now
Person Y: Dunno, Schrödinger's Cat's in the bag
Person X/Y once rumour has gotten around: Guess Schrödinger's Cat is out the bag now