If you right click a game either in your Library, or on the game icon like this for any game, you go to "Properties". The next window that pops up will have a "Privacy" option. If you turn on "Mark as Private" the game will not show up on your 'currently playing' for friends, it will not show that you own it when a friend views the games steam page, hides achievements, and if someone buys the game it will show up that they can still gift it to you even though you own it, you just have to hit the decline button to send it back. It will 100% make it hidden to anyone outside of you logged in that you don't own the game. Marking it as hidden will mean that if someone has access to your steam account and they look though your library, it won't show up there either. Useful for program editing things like Skyrims Script Extender, you'll just have to go to filter games at the top of your library list and click "installed games", and it will show up at the bottom as "hidden." Enjoy.