I posted this in another thread somewhere, but there are only 3 people who could have told Hook about what happened: Zelena, Belle, Lumiere.
Lumiere: We don’t know where he ended up, but as an inanimate object, he wasn’t running around in such a way that you’d expect Hook to run into him, especially if Hook was headed for where the water is, and Lumiere is landlocked.
Belle: We’ve been led to believe, by Hook’s own admission, that the last time he saw all of them was when he left them in 312. Also, given Belle’s distaste for Hook in 315, I find it hard to believe that she’d have confided anything in him if she’d encountered him in FTL.
Zelena: I know there’s been speculation about her being the one to have sent him with the potion, but that doesn’t make sense to me that she’d send Walsh to keep Emma away, and then send Hook to bring her back, so I don’t really think Hook and Zelena interacted back in FTL.
The other option of course is that Hook was hiding in the trees and watching what happened. We don’t know how much time had passed by the time Belle and Neal got to Rumple’s castle, and we don’t know FTL geography to know if that clearing in the woods would have been on Hook’s path if he was still headed towards the coast to find his ship at that time.