Actually, blues have stated that Sigils/Secrets/Runestones are on a separate loot flag. If you didn't have the quest when you looted the boss, you can potentially get them. If you had any of the related quests when you killed the boss, you can't get them.
Players have also tested and proved this to be true.
But ya, if you've already killed the boss and had the quest in your log, no more chances that week. True across all difficulties/lockouts.