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While Robin and King Neptune discuss privately about Shirahoshi, confirming her identity for the first time as the ancient weapon Poseidon, they believe they are alone. They state everything openly, and Neptune even speculates how terrible it would be for the information to get out to the world. This all happened before the Reverie, though it seems that Imu perhaps has an idea that Shirahoshi is Poseidon because of her photo, she did not choose her, rather selecting Vivi who is the one with access to Pluton.
Caribou, unfortunately, was hiding beside Robin and Neptune listening to their conversation. It is the only truly significant secret he is shown to discover, framed by Neptune's comments about the tragedy it would be for the news to leak. Caribou is now headed to Blackbeard.
So, Blackbeard will be heading to Fishman Island. That gives him possession of Garp, Pudding, and Shirahoshi, more than enough motive for him to be the Strawhats' next target.
Cross Guild is going to Impel Down to finish rounding off their crew.
Shanks is going to Raftel.
Blackbeard is going to Fishman Island.