Oi! Thanks a lot!
This is indeed the mapping tool I had used years ago. However, it looks like my memory was quite hazy, too.
Unfortunately, it doesn't work quite es well as I had remembered. BUT thanks to some additional search results Google provided I found
IFMapper.
And THAT does provide automapping.
It essentially works by analyzing the transcripts of Infocom-games (a text file the game creates when using the "SCRIPT" command in game).
And, really, it works rather well. It auomatically adds rooms, marks your current room and even tries to guess possible exits according to the room description.
And, of course, it realigns rooms constantly so the map makes sense.
It even adds items you pick up or drop to the room description.
Of course, there are limitations. It won't work "in a maze of twisty little passages all alike" as each room has the same name and description.
And it does add "pitch black" rooms as separate rooms once you light a lantern.
All in all it's a great tool and something that really enhances gameplay for me.
The only downside is the installation: It uses ruby and might be a little strange to get it to run.