Game removed from Steam Library permanently

The main reason they'll remove a game is if the purchase was revoked so it was likely something you traded for.

Get any great deals on a game lately?

I sugest you to make an account on IsThereAnyDeal, they keep an eye on your library and send you an email if something disappear from it(and also tell you about sales and other stuff). You could also check https://store.steampowered.com/account/ to see your games.

Usually if it's a game you bought or traded for it'll give you a green banner telling you what was removed.
I too had one less game today but I think it was a beta that was removed.

If you haven't synced ur account with steamgifts yet after that hppened, it may hold your gamelist in its filter...

If u synced or u didn't synced after u got that game... Move along! =P

it could have been a free weekend game. those appear in your account and then get removed after the period is over (i dont know what day you last looked at your games count).

steam usually has a popup when they do this telling you what happened. if it was a free to play game or a beta or something though they wont do this. if its a game with a price then they tell you, like i won surgeon simulator from steamgifts and a week later the game was removed cause the guy that did the giveaway either used a stolen card or didnt have enough money to pay and i got a popup from steam telling me it was removed due to payment issues. its not something you can miss either as it pops up right in your face

Check your Civilization IV games, normally there should be two of each as the Mac versions are considered a different game. I mention this because my second version of Civ IV Colonization dissapeared a few hours ago.