My first thought when she found something was that Gold had left it there since he was the last one in the cell (as far as we know). But since they made a big deal about the keyring with keys to every lock in Storybrooke, that points to the key being off of Regina’s keyring. But remember, the last person we saw with that keyring was Henry, and we know he wants to help MM, and we know he spends time at the sheriff’s station.
So my list of suspects is Regina, Henry, and Gold, with Henry now my leading candidate.
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