I don't remember buying this.

[Spark7979]

I have this in my library, but I do not remember buying it. Did I get it as part of a bundle by mistake, perhaps?

[Calmly Frenetic]

On the Steam client window, click on your wallet balance ($) at the top right of the screen. From the Account Details page, click on the link for "View licences and product key activations" to see if you can track down when/how you got the game. For bundles, click on the link for "View purchase history" and click on the bundle to see the individual items.
Enjoy playing, especially if you got it for free.