[Melusine]

The code of the key you lost is MISC4S.
If you don't have ShadowKeeper, simply do the following:

Do you have cheats enabled? If not, go to your BG2 folder, and find a file called Baldur.ini (make a backup of it in case anything goes wrong) Open the file, and under Program Options, type the following line:

Debug Mode=1

Now, save the file and start up BG2. When in your game, press Ctrl + Space to bring up a small window at the bottom of your screen. Copy the following into it:

CLUAConsole:CreateItem("MISC4S")

Now the key will be in your inventory again. There is no time limit in which you should use it.
Hope this helps.