Q:
I've found the key to the safe box after I listened at the door. But now I can't find the safe box itself. Where is it?
A:
It's not there. Your mission is to make a copy of the key and leave with it, so stick to that... you will find the safe later in the game.
Q:
How do I make a wax impression of the Key?
A:
Behind the work shop in the cellar, there is another room (on the right) where you find appropriate material and tools for this task.
Q:
Where do I return the Safe Box Key to?
A:
To the very place you picked it up from.
Q:
Where do I put the key back down?
A:
Depends. There are several possible places; it varies from game to game. You need to pay attention to Garretts commentary during the game; the places are chosen randomly upon mission start... and taking a note after you heard Karras tell his brethren where they can find the key is also a jolly good idea.
Worst case you need to load your saved game or start the mission over. Or try all of the places where you found a golden key.