Reasons I think Smee is spying for Zelena:
1. Multiple references to him being a rat. (And by multiple I mean overstated - seriously if all they wanted to do was address the transformation back to human they did it in the robbery scene. They didn’t need to keep mentioning to it every ten minutes)
2. Smee was giving Killian a pretty measuring look when he said that things were wrong because he didn’t have HER. Pretty sure he wasn’t buying “the Jolly Roger is my true love” story. Smee knew something was up. Also I would not put it past him to have used his time as a rat in SB to spy on people and it would have been obvious to him that Killian had feelings for Emma if this was the case and hence the look of “not quite buying it” I saw on his face in this scene.
3. Smee was again present when Killian mentioned his reasons for staying in Storybrooke were his own, right after Emma and Henry come into the scene. That little schemer is not so thick as to not notice Killian’s obvious motives here.
4. Smee is probably really great at reading the people around him, if for nothing else than knowing when it’s time to cut and run in order to save his own hide. He knew full well what selling out Ariel was making a man like Killian feel. This is a man he’s spent countless years with - he knows him we’ll enough by this time that he’s seeing the good man shining thought that vicious pirate veneer and was in a perfect position to observe the ensuing guilt.
4. Smee looks out for Smee above all else. It would be well within his character to be working for Zelena if she threatened him or promised him something. Loyalty is not a strong part of his vocabulary. Pretty sure Smee would sell out his own mother if it benefitted him.
I wondered about Smee too, and these are all great reasons why he is very likely to be a Zelena-spy. I would also add, kind of as an offshoot of #4 (4A, if you will), that Zelena said she knew that Ariel had found Eric on her own out on Hangman’s Island because her “spies are circling everywhere.” That would imply the monkeys (circling in the sky) had seen their reunion…but without having context for why the manner in which Ariel found Eric (i.e. finding him on her own) was important, I don’t think Zelena would have known that she could use Ariel’s face/form to turn the screws of Killian’s guilt. I think she had to have known that Ariel had asked Killian for help and he’d decided not to help her and that that was why she showed up on the Island on her own….I think Zelena would have had to have known the whole story in order to use any of it to play Killian, and just seeing Ariel swim up to the Island wouldn’t have given her the full picture.
So considering how else she could have gotten the full picture, the options are limited - it would have had to have been from someone on board the Jolly Roger when Killian chose to kill Blackbeard and Ariel made her declaration and left. That was Ariel, Killian, Blackbeard, Blackbeard’s crew, and….Smee. Ariel and Killian would never have told Zelena, Blackbeard’s dead, and the anonymous crew members wouldn’t logically be that important in the plot….so, yep, I smell a rat.