Yesterday, I was playing a game on steam, and I noticed that I had Dota 2. I never bought the game, and I was never even interested in it. It is $29.99 USD and I haven't spent that much on a single game. Most of my games were free or less than $10.00. Anyway, I never downloaded a free trial and steam never gave me a free trial for 3 days. When I was looking at more information on the game, I noticed that all of my friends on steam had it too, even though they never bought it either. Has anybody else noticed this?
Dota 2 is a free to play Moba currently in beta, the 29.99 is to get beta access. More than likely someone sent you a beta key, Valve gave me a dozen of them to give people.
I think unless you added it to your library you can still do so or add it to your gift inventory to regift. It could still be in limbo, waiting for you to accept.