When Stelly came to the throne room the knights told him that no one sits on the throne and that it symbolises equality in the WG nations, meaning most people in Mariejois don't know about Imu's existence I think the secret treasure is Imu because they showed him/her right after the scene where Doffy talks about how he knows the secret in Impel down. But they showed the strawhat in a vault so you may be right. Yeah that sounds reasonable. I also think that Sabo is on the loose with Vivi after they assassinated Cobra. I still don't understand why Shanks is so worried about BB. He even went to WB to stop Ace, and now this. The whole situation seems very weird. Yeah must have something to do with Ancient Weapons. The sword was in Shirahoshi while Luffy and BB's posters were cut.