Secret was a game played on the British and American editions of Whose Line.
Always played by Ryan Stiles and Colin Mochrie, the scene would be given by Clive Anderson. The audience would then be asked where "a secret" might be discovered, given the scenario. A scene in the Notre Dame bell tower with Ryan as Quasimodo, for example, had the secret in the bell itself. The two performers would then act out the scene "twenty seconds before the secret is discovered", traditionally with Colin discovering Ryan's secret. After the discovery, the scene would continue with this knowledge.
The secret was always prefaced by the secret-keeper's reaction of "I didn't want you to see that" and the finder's shocked expression, at which point the discoverer would traditionally announce the secret as an outlandish item. The above scenario in Notre Dame had Quasimodo hiding a tuba, while one set on Noah's Ark had extra commandments being hidden. In some situations, Ryan (a famously straight-faced performer) would be visibly amused by Colin's suggestion.
Secret started appearing on the American Edition in the later seasons. Starting Season 10, the secret would become a prop stored in a chest. Colin and Ryan would then continue the scene, improvising with the prop.
Series 7: 6, 10
Series 8: 2, 3, and 5
Series 9: 6, 7, 15, and 17
Series 10: 2 and 4
Season 8: 11 and 15
Season 10: 8, 10, 18, 20, 23
Season 11: 10, 16