Ve hře Zaklínač, klíče are almost exclusively, but not entirely příběhové předměty. As such, they are placed in Geralt's quest item inventory pocket and they stay there, safely. They cannot be lost which is very handy. There are quite a few keys in the game and many of them look alike, hopefully the list below will help make sense of them.
Key to Beraninův dům in the Chrámová čtvrť slumach, which you can acquire by killing his guards at night (apparently, he isn't bothered by this in the least). This is the only key in the game which does not go into the quest items inventory pocket, and so, could actually be lost or accidentally dropped. Of course, depending on how you make your choices in this game, you may never need this key, and you can just pay the guards, but that's each time you go.