I’m not exactly sure what Hook is hiding but I think it may involve Ariel
Now we know that the scenes between Hook and Ariel are going to be tricky as Colin said in an interview or something to that effect.. I’m betting the bird and message was from Scuttle. What if Hook has asked Ariel to do something for him or they came up with it together but she has to go some place far far away to get this object or whatever as she can travel realms. This object or whatever may help them fight against Zelena and Hook may know of this object because we know he has been at sea and could have come across a story or overheard something about it and that is why Ariel is on a mission and he is hiding something as Ariel will bring this item back to Storybrooke with her.