That's not so uncommon. I activated my SR3 DLC pack before I activated the SR3 game in my Steam client.
I think I've got another DLC currently in "Steam Limbo", which I got from a chat giveaway (I think it was a Dwarfs DLC).
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http://store.steampowered.com/app/8650/
While it may appear as a free DLC for only those who own the full game - actually anyone can activate it, even if you don't own the base game.
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