Can Thief steal what you don't see?
The thief does move around on his own, and will pick up objects you have dropped from a location where you are not currently yourself. The thief can also randomly drop items at different locations, thus in effect moving items. This can confuse maze solving, if you are using the old drop-an-item-in-each-room trick to identify the rooms.
I think that the quote "only takes things which you have seen" refers to the fact, that all items in the game are supposed to be found in a specific location, so the thief will not move them until you have "found" them.