Caribou heard Robin and Neptune talking about the Ancient Weapons and learned that Shirahoshi is Poseidon. During Caribou's cover story, Caribou was causing problems at Kaido's favorite island and was captured by X Drake, who we know is a member of Kaido's crew. Caribou will likely tell Kaido about Shirahoshi's secret in order to avoid punishment from Kaido. If this happens, Kaido will likely start a war against Big Mom, Fishman Island, and the StrawHeart Alliance in order to capture Poseidon. It would be a win-win scenario for Kaido since he would either capture Poseidon or die.