Interpreting the Past Past Perfect Tense

Hello everybody,

Here's a sentence that I'd like to discuss: «The other snarled aloud into a savage laugh; and the next moment, with extraordinary quickness, he had unlocked the door and disappeared into the house.»

At first I was baffled by the choice the Past Perfect for "had unlocked", but then I came up with the interpretation that it was used to emphasize the 'momentariness' of the unlocking of the door. It was done so quickly that in the next moment it had (already) been unlocked, which is one of the standard situations to use the Past Perfect in.

Does this explanation make sense to you?

Anton