Occasionally I notice I gained 2 reputation for no obvious reason. Presumably someone has just changed their mind about a previous downvote and reversed it, but they didn't leave a comment about it. Is there any way to see where this happened?
Reputation can also be "gained" if you downvoted some answer that later got deleted: you get the reputation point back. Two of these, and you get 2 reputation.
Another option is that answer of yours that had a downvote got deleted.. again the reputation is given back.
Downvote can be undone only 5 minutes after being cast, so it's unlikely that you spotted it on time.
Short answer: No, it is not possible.