@opencage The first one of these I learnt of was the Elephant contour line, seen in a map exhibition at The British Library in the early '80s. Covered here by Strange Maps: https://bigthink.com/strange-maps/670-nil-how-to-hide-an-elephant-the-1923-gold-coast-survey/ I'd missed the Swisstopo ones but there's a good collection on the Ordnance Survey map of the Isle of Wight: mainly draughtsmans' names hidden in cliffs.