Can Thief steal what you don't see?

I'm now going through Zork I (revision 88, Infocom release) and mapping the maze of twisty little passages by dropping various stuff here and there.

What is thief tells "Since he steals for pleasure rather than profit and is somewhat sadistic, he only takes things which you have seen", but "things which you have seen" is not the same as "things that you currently see" to me, and to my understanding I've seen all the things I've dropped in the maze, because I possessed them.

Question is — if I drop a sword, or painting, or egg, or something else valuable somewhere in the maze, could Thief steal it while I'm not in that room? In other words, does Thief move on it's own, or it does not exist unless stars align?