In Germany "00" stands for the toilet. https://www.wissen.de/warum-nennt-man-die-toilette-auch-00 When large Intercontinental hotels were first built in the 19th century, the operators numbered each room. However, the toilet was (and still is) not considered a room in bed castles, so it was given the number “00”. Since then, the double zero has become the standard designation.