Regular commenter Dmitri remembered that he'd seen a sign for the Tube in Luxembourg
However, the weirdest came from Ellen F who spotted this advert on the Hauptbahnhof U-Bahn (central station underground station) in Hamburg.
My German is rubbish but it seems to be something about some adult cinema. It certainly would have looked good against the next advert. Mike from Toronto saw this in Dehli, and although it's not another roundel it's a clearly "borrowed" from the Tube's map design
If you want to see the station names click on the image to err, enlarge. I can just about make out some saucy Carry On names such as Mile High Airport, Threeway Junction, Lower Grind, Seemore Street, Spanky End and Moor Wood. They must have had a lot of fun in the creative department of that agency.
Keep those sightings coming in and I'll add to the collection.