когда рак свиснет => когда рак [на горе] свистнет
Свистящий рак нашли? => Свистящий рак найден? или Нашли свистящего рака?
I think people would generally be puzzled by that question, but eventually they'll figure it out even though that is not 100% accurate. The original saying "never" is: "когда рак на горе свистнет", so the question of had the highly improbable happened would sound like: "Ну что, рак свистнул?"