I have someone's game in my library.... (???)

Contact Steam support about it, we're completely powerless.

This shows up if somebody else uses that steam install (on the same computer) also with their own account. Steam shows then their games that are installed but you don't own them. Just right click and delete local data as always, will remove the game and it will stop shwing up on your account.

I have something similar in my library The Showdown Effect Beta Strikes Back. It appeared after that update everyone is complaining about.

Scary shit, I think someone uses your computer for their steam account without you knowing, or you have split personality and its the game of the other you.

Wait.... is that what I think it is?

click

Oh, so that's where my game went, I will take it back now -.^

Someone must installed this game to your computer via his account. There is no other explanation. Search your computer for save files or some other remaining stuff if you deleted local content. It must be there.

Unless you can right-click on the entry and delete local files, you'd need to log in as the other user, and uninstall the game from there.

As others have said, it's because another person has been logging into their account on your PC.

This seems more like the prelude to game sharing than anything else. When I had someone else using my PC, their games just appeared in the middle of my library. I just couldn't play them.

it usually only does that if it is installed afaik ...

Wow... That's really odd... O_o

I've noticed it's a bit odd about what shows up, for instance the Hitman sniper training or whatever that comes with Absolution shows up in my library but not the full game.