I had never considered this possibility. If he was, why? I doubt it is his heart, because that would give Regina far, far too much power over him and Mary Margaret. If it is Kathryn’s, then maybe something happened in FTL and he is Charming looking for Abigail’s heart. I doubt that, though.
This is an interesting theory, and explains why they show us David’s problem again in this episode, before they find the box, but I don’t think this is the case. I think the box was planted by Either Regina or Mr. Gold in an attempt to either frame Mary Margaret or accomplish something else. Mr. Gold may want to force Emma’s hand, and make her look deeper into what is going on. But I still just think it is Regina framing Mary Margaret.