[Shadowhal]

Voici la réponse de Wizards concernant une question que je leur avais posée. C'est pas mal plus clair ainsi.

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If the door is locked then it is possible that the creatures in the room hear you unlocking it. It is up to the DM to work this out, as it seems he has done. But, if they haven't acted in combat yet, then they are considered flat-footed, so the rules are on your side. Once you roll initiative, if you go before the enemy, then if you make an attack on one of them in the first round, they are considered flat-footed and you should get your sneak attack bonus. The flavor of this could be explained by the fact that while they may be ready for a combat, you are still acting before them, being a bit faster, and catching them a little off guard. So again, if you act before them in the first round of combat, they will be flatfooted and you will get your sneak attack damage, assuming they can be effected by sneak attack.