In The Witcher, keys are almost exclusively, but not entirely quest items. As such, they are placed in Geralt's quest item inventory pocket and they stay there, safely. They can not 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 Ramsmeat's place in the Temple Quarter slums, which you can acquire by killing his guards at night (apparently, he is not 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 (20 ), but that is each time you want get inside.