Are you entirely sure you picked it up?
it's not on any of your characters. Maps will begin to disappear if you begin a monster fight on the last tile or start moving after you've explored the last tile.
You can usually find it where your fish/fishing rods are.
Was there anyone else in the game? as it's possible it went to the other player (in group games, if multiple people click an item, it basically rolls who gets it so everyone has an equal chance).